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Dive into Mozilla Modifying Firefox using an Extension Lab

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[[Dive into Mozilla]] > [[Dive into Mozilla Day 5]] > Modifying Firefox using an Extension Lab
''' progress...'''
In the [[Dive into Mozilla Modifying Firefox Lab|previous lab]] we made a small change to the behaviour of Firefox by modifying the browser's source code. In this lab we explore how to achieve the same effect using an '''extension''' rather than modifying the tree. ''(Thanks to [ Mark Finkle] for the idea of redoing this as an extension.)''
The goal of this exercise is to expose you to Firefox extensions and to show you how to modify or extend the browser without changing it's its source code directly. Some thought will also be given to the two methods of doing this (i.e., in the tree vs. as an extension), comparing their advantages and disadvantages.
=The 'What': write a tab creation extension=
The first problem with doing things in the tree is that in order for you to distribute the local changes you've made, people will have to use your custom build of Firefox. While this might seem like a good idea the first time you build the browser, it isn't sustainable long term, and users will always want to get the browser from Mozilla for security fixes, new features, etc.
A logical alternative might be to try and get your change accepted into the tree. This involves filing a bug on and then creating and attaching a patch with your changes. The problem here is that even though you (and I) might think it is a good idea for tabs to be created in the way we've specified, the community may not--people have already put thought into the way things work now, users are accustomed to it, etc. In this case your bug will likely be marked '''WONTFIX''', which means that your patch won't make it into the tree.
What does this leave? You could [ fork] Firefox, as some people have done, and create your own version of the browser. Obviously this isn't what we'd like to do. Rather, what we need is a mechanism to insert a small change into the browser, and do so in such a way that users can choose to install our code or not. Mozilla provides such a mechanism in the form of Extensions.
Extensions allow third-party developers (and Mozilla, for that matter) to write add-on packages that users can install to extend or modify the standard browser. By rewriting our earlier code as an extension, we can give our users an extension to installa small add-on, which will have the same effect as our custom build. For all but the most universal of changes, extensions are the best way for developers to write code that targets the browser.
==Planning the extension==
Having decided to rewrite the code as an extension and remove our changes from the tree, a logical first thought would be: "how can I modify my existing code to create an extension." While we learned some useful things modifying the browser directly, our extension will require a completely new approach to solving the same problem.
Having already successfully made this Instead of searching for existing code to change once before, we have to take the browser as a logical first thought would begiven and find ways to work with it. To that end, let's look again at the code for [http: "How can I modify my existing code to create an extension//" xml#1122 tabbrowser's addTab] method:
var t = document.createElementNS("", "tab");
// Dispatch a new tab notification. We do this once we're
// entirely done, so that things are in a consistent state
// even if the event listener opens or closes tabs.
var evt = document.createEvent("Events");
evt.initEvent("TabOpen", true, false);
return t;
Start by creating a After the new tab is created and appended to the list, an '''event''' is [ dispatched] to the new extensiontab element. The developers of tabbrowser.xml wanted their code to be extensible; that is, either they wanted to provide a way for future developers to extend the code by handinserting a hook. In this case, or using Ted Mielczarekafter a tab is created an opportunity is provided to do 's wonderful wizard: 'something''. Their foresight makes our job much easier. to move a tab?===
Now we know that there is a logical time/place to run our code via the '''TabOpen''' event. The next thing to consider is what our code will do. Previously we replaced '''append''' with '''insertBefore''' and stopped the problem before it happened. However, in this case by the time the '''TabOpen''' event is dispatched, the tab will already be created and positioned at the end of the list.
Directory structureWe need another solution. It's time to go hunting in tabbrowser's [ documentation] and [ code]. What we need is a way to reposition a tab after it has been created. A quick look through the docs for [ tabbrowser] reveals nothing. However, the [ code] is more helpful:
'''addtabbeside <method name="moveTabTo"> <parameter name="aTab"/'''> content.manifest <parameter name="aIndex"/> install .rdf '''content/''' firefoxOverlay.xul overlay.js
The '''moveTabTo''' method takes a tab to be moved, and a new position. This is exactly what we need. Now there are just two things left to figure out: 1) how to get the newly inserted tab; and 2) the index of currently selected tab so we can go one to the right.
Here is ===How to get the '''content.manifest''' file:newly created tab using code?===
content addtabbeside content/ overlay chromeHaving already studied and worked with the code in '''addTab''', and knowing that new tabs are always appended to the end of the list, we could do the following in order to get the new tab://browser/content/browser.xul chrome://addtabbeside/content/firefoxOverlay.xul
// In an extension, gBrowser is a global reference to the tabbrowser element
var container = gBrowser.tabContainer;
var lastIndex = container.childNodes.length - 1;
var newTab = container.childNodes[lastIndex];
However, looking back at the event dispatching code for '''TabOpen''' we see that the event is dispatched to t, where t is the newly created tab:
Here is This means that in the event system, we'll be able to listen for and capture this event, and in so doing get at the tab object via the event's 'install.rdf''target''' file for the extension. Three lines of code become one:
<pre> <?xml versionvar newTab ="1e.0" encoding="UTF-8"?> <RDF xmlns="" xmlns:em=""> <Description about="urn:mozilla:install-manifest"> <em:id></em:id> <em:name>Add Tab Beside</em:name> <em:version>0.1</em:version> <em:creator>David Humphrey</em:creator> <em:description>New tabs are created beside the current tab instead of at the end of the tab list.</em:description> <em:targetApplication> <Description> <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id> <!-- firefox --> <em:minVersion>2.0</em:minVersion> <em:maxVersion>3.0a3pre</em:maxVersion> <!-- trunk build Feb 27, 2007 --> </Description> </em:targetApplication> </Description> </RDF></pre>target;
We'll discuss this code in more detail below.
Here is firefoxOverlay.xul:===How to get the index of the currently selected tab?===
<pre> <?xml version="1.0" encoding="UTFWe know that in order to move a tab using tabbrowser's '''moveTabTo''' method we need the tab object--which we now have-8"?> <overlay id="addtabbeside-overlay" xmlns="and the index where it should be placed. Looking through the tabbrowser code again, we see ["> <script src="overlay.js"toolkit/> <content/overlay><widgets/pre>tabbrowser.xml#1453 references] to code like this:
var currentIndex = this.mTabContainer.selectedIndex;
Load Listener codeObviously we can't use '''this''' outside the context of tabbrowser, so in an extension we use '''gBrowser''' instead to get a reference to tabbrowser:
If we want to get the tab to the right of the current tab, we simply do this:
var positionPlusOne = gBrowser.tabContainer.selectedIndex + 1;
Reading through Are we done? Not quite. This code will work, but the rest problem we now face is that our code will run ''after'' the tab has been placed at the end of the list, when '''moveTabTabOpen''' method, we see that when a is dispatched. As soon as the new tab is successfully created, it becomes the active tab. That means that '''TabOpenselectedIndex''' event will always be the index of the last tab, and moving the last tab one to the right is dispatched nonsense.
(see What we ''really'' need is the position of the tab that we were on when he made the new tab. We even know how to get this information, using '''tabContainer.selectedIndex'''. What we don't know is how to get this information, since it is lost by the time our code runs.)
// Dispatch a new ===Saving tab notification. We do this once we're // entirely done, so that things are in a consistent position state // even if the event listener opens or closes tabs. var evt = document.createEvent("Events"); evt.initEvent("TabOpen", true, false); t.dispatchEvent(evt); return t;==
This is useful information, because it tells us It's clear that if we want to know when a need data from the past--the position of the tab we were on before this new tab has been was created. We can't go back in time, so we 'll have to add store this data in a listener variable for this eventinspection later.
In order to accomplish this, we need another event that tells us when a tab has become the active tab so we can store the position state. A quick search through tabbrowser for other events (e.g., search on '''"dispatchEvent"''') shows these possibilities:
moveTabTo* TabSelect* TabOpen* TabClose* NewTab* TabMove
http://lxrThe first event looks interesting.mozilla All we need to do now is have a variable that gets updated with the value of '''''' every time '''TabSelect''' occurs. Then, we can use this variable's value to calculate the desired position in '''moveTabTo'''.xml#1958
Finally, we've got all the pieces in place and can write some code.
Create a file named: ''' 'How': the extension's code=
This file should contain Now that we have the full path logic for our code, all that remains is to write the necessary pieces in order to your have it get executed at the right time. When we modified the code in the tree this wasn't a concern: we assumed that code we changed in '''addTab''' would get executed whenever '''addTab''' got called. With an extension, for example:we have to explicitly tell the browser how and when to execute our code. We do this using '''event listeners'''.
From our previous research, we know that the methods in tabbrowser cause a variety of events to get dispatched into the event system. C:\temp\addtabbesideThese events move through the event system regardless of whether any code notices them--they are passive. In most cases, nothing is done in response to events. However, any developer can decide to do something in response to an event, which is what extension developers must do in order to get their code into the browser.
Now put this file in your development profileWe need to wire our code to a number of events by [ adding event listeners]. Two of the events we know already, ''s extensions directory (NOTE: replace 'TabOpen'Username'' with your username and ''dev'TabSelect'''. However, we also need to register our listeners at start-profileup using the '''window's load event'''. We' with your development profile name):ll add code to remove our event listeners in the '''window's unload event''' too.
C:\Documents and Settings\''Username''\Application Data\Mozilla\Firefox\Profiles\''dev-profile''\extensions==addtabbeside.js==
      To keep things clean we'll write all this code in a custom object called ''overlay'AddTabBeside'''. Here is '''addtabbeside.js''':
var AddTabBeside = {
onUnload: function() {
// Remove our listeners
var container = gBrowser.tabContainer;
container.removeEventListener("TabOpen", this.onTabOpen, false);
onTabSelect: function (e) {
// when When a different tab is selected, remember which one. This is
// necessary because when a new tab is created, it will get pushed
// to the end of the list, but we need to know where to put it.
// Insure that our code gets loaded at start-up
window.addEventListener("load", function(e) { AddTabBeside.onLoad(e); }, false);
This code will become part of an overlay that will be merged with the browser at runtime. But where do we put this file? Firefox needs to know where it is in order to load and run it at startup. We also need to create an installation package for our overlay.
==Creating the rest of the extension==
Firefox extensions are packaged as compressed zip files with a '''.XPI''' extension. This is what you download when you visit and choose to install an extension. Because extensions are just .zip files, you can unpack any extension and see how it's built. For example, try saving [ ChatZilla's .xpi] to your computer and decompress it (NOTE: if your built-in unzip program won't do this, try changing the file's extension to .zip).
An extension is a series of files and directories containing JavaScript, XUL, CSS, XML, RDF, and other custom text files. Some of these files define scripts to be executed, such as those we wrote above. Other files contain information ''about'' the extension, metadata telling the browser how to integrate and install things, what the extension is called, its version number, etc. We need to create these other files now.
===Extension files and directory structure===
Start by creating the following directory structure:
Because this is a first extension, we will skip some [ other directories and files] that more complete extensions would include (e.g., localization, skins).
The first thing to do is to copy the '''addtabbeside.js''' file we wrote earlier to '''addtabbdeside/chrome/content/addtabbeside.js'''.
This code now needs to get merged into the browser so it can access elements within the application. We do this by providing a [ XUL Overlay] file. A XUL Overlay is a .xul file that specifies XUL fragments to insert at specific merge points within a "master" document. Often this is used to add new UI to an application (e.g., a new menu item), but in our case we'll use it to merge our addtabbeside.js script into the browser.
ItWe need to create 's one thing to say you'd like to change the browser's behaviour, but quite another to actually do itaddtabbdeside/chrome/content/overlay. The change you have in mind might be quite simple, in the end (ours is). But you still have to figure out where that simple code needs to go. That can be difficult. However, difficult isnxul'''t the same as impossible.
How do you begin<pre><? First, let's start at the top and find some UI notation we can search for in the codexml version="1. In our case, we can focus on the various methods for creating a new tab0" encoding="UTF-8"?> <overlay id="addtabbeside-overlay" xmlns=""> <script type="application/x-javascript" src="addtabbeside.js"/> </overlay></pre>
* CTRL+T* Right-Click an existing tab and select New Tab* File > New Tab====addtabbeside/====
The second Having added our script and third methods are useful, as they provide us with a unique string we can search for in the code. Before we can change anythingoverlay files, we have now need to search and read existing code in order add a couple of metadata files to help Firefox understand where to begin--this is the standard pattern for open source and Mozilla developmentinstall/load our extension.
==Search 1 - finding The first is '''addtabbeside/chrome.manifest''', which is a UI string==[ Chrome Manifest]. This file helps Firefox translate between chrome:// URIs and actual files on disk. It also allows us to specify where our overlay will be merged into the browser:
We # Chrome package addtabbeside has it're looking for a unique string--"New Tab"==, so we'll use [http:s content in ./chrome/content content addtabbeside chrome/content/lxr # Overlay the browser.mozillaxul file with LXR's] '''Text Search''' featurexul overlay chrome://browser/content/browser. Here are the results you get when you search for "New Tab"xul chrome://addtabbeside/content/overlay.xul
<blockquote>httpThe first line registers the location for our content (i.e., .xul, .js). The second line registers our overlay, and says that overlay.xul will be merged with browser.xul. Mozilla uses chrome://lxr.mozilla.orgURIs to refer to aspects of the interface, and chrome://seamonkeybrowser/search?string=New+Tab<content/blockquote>browser.xul ''is'' the browser (try typing it into the address bar).
Lots of results, many of which point The second metadata file we need to comments in the codecreate is '''addtabbeside/install.rdf'''. HoweverThis is an [ install manifest] that tells the Firefox add-on manager how to install the extension, with information like who wrote it, the first result looks interesting:version number, compatibility information, etc.
<blockquotepre><?xml version="1.0" encoding="UTF-8"?><RDF xmlns="" xmlns:em=""> <Description about="urn:mozilla:install-manifest"> <em:id></toolkitem:id> <em:name>Add Tab Beside</localesem:name> <em:version>0.1</enem:version> <em:creator>David Humphrey</em:creator> <em:description>New tabs are created beside the current tab instead of at the end of the tab list.</em:description> <em:targetApplication> <Description> <em:id>{ec8030f7-USc20a-464f-9b0e-13a3a9e97384}</chromeem:id> <!-- firefox --> <em:minVersion>2.0</globalem:minVersion> <!--<em:maxVersion>3.0a9pre</em:maxVersion>--> <!-- trunk build Oct 14, 2007 --> <em:maxVersion>3.0+</tabbrowserem:maxVersion> <!-- work for v3.dtd#20 and above --> </Description> </em:targetApplication> </Description></RDF></blockquotepre>
Here we see ===Testing the DTD file describing the key/value pairs for the en-US localized strings. Mozilla uses this technique to allow localizers to translate strings in an application into many different languages without having to change hard-coded strings in the code (you can read more about localization, DTDs, and Entities [ here]) extension===
Looking closely at Eventually we'''tabbrowserll package our extension properly into a redistributable .xpi.dtd' However, while we're testing it' s nice to be able to use it in an expanded form so we see that our English string, "New Tab", has the following entity: can make changes.
<!ENTITY newTabTo this end, create a file named '''addtabbeside@senecac.label "New Tab">''' and put it in your development profile's extensions directory (NOTE: replace ''Username'' with your username and ''dev-profile'' with your development profile name):
This is good information, because it allows us to repeat our search with an entity instead of a string, which should help us get closer to the code we C:\Documents and Settings\'re after'Username''\Application Data\Mozilla\Firefox\Profiles\''dev-profile''\extensions\
==Search 2 - finding an ENTITY==Or in Mac OSX
Repeating the search with the '''newTab.label''' ENTITY value instead of the "New Tab" string makes a big difference--we have many fewer hits: ~/Library/Application Support/Firefox/Profiles/
<blockquote>httpThis file should contain a single line of text--the full path to your extension, for example://</blockquote>
Not surprisingly, the first result is the same DTD file (i.e., tabbrowser.dtd) we already found. The second result looks interesting, thoughC<blockquote></blockquote> Here we see the code to generate the pop-up context menu for a tab (i.e., what you get when you right-click on a tab in the browser):  <pre> <xul:menuitem label="&newTab.label;" accesskey="&newTab.accesskey;" xbl:inherits="oncommand=onnewtab"/> </pre> Having found the appropriate entity value, we also notice the use of a function name, '''onnewtab'''. This line of code says that the xul:menuitem will inherit the '''oncommand''' value from its parent (you can read more about XBL attribute inheritance [ here]). In other words, when this menu item is clicked, call the '''onnewtab''' function. ==Search 3 - finding a Function== Armed with this new information, we are even closer to finding the right spot to begin working. We've gone from UI string to XML ENTITY to function. All we have to do now is find that function: <blockquote></blockquote> This returns many results for things we aren't interested in, including files rooted in /suite, /db, etc. Since we are interested in finding this behaviour in Firefox, we need to focus on the files rooted in '''/browser'''. One looks particularly interesting: <blockquote></blockquote>\temp\addtabbeside
In this case, the tabbrowser widget has the onnewtab property set to another function, Start your browser and make sure your extension is loaded (check '''BrowserOpenTab();Tools > Add-ons''' (i.e., Firefox seems ) and working properly by creating some tabs to handle tab creation in a non-standard way, providing its own method instead of using the default)see where they get positioned. Since we want to find If you're having problems, check the definition of this function, we search for '''"function BrowserOpenTabError Console''' ("''', which returns two results:Tools > Error Console''') for hints.
<blockquote></blockquote>==Packaging the extension===
AgainOnce debugging and testing is complete, weyou're interested in Firefox (ill want to [ create an installable .expi file] to give to your users. As was previously mentioned, browser) instead of SeaMonkey .xpi files are just .zip files with a different extension. All .xpi files must contain the '''install.rdf''' file in the root (i.e., suite)when you unzip it, so we skip to install.rdf should be in the second result:top directory).
<blockquote>httpFollow these steps to create an installable .xpi://</blockquote>
This shows us that we need # Create a new zip file named '''''' (NOTE: you can use .xpi instead of .zip if your application will allow it).# Add your extension files and directories to be looking for yet another function'''''', making sure that '''install.rdf'''loadOneTabis in the root (i.e., don't zip the addtabbeside directory itself, just it's contents).# Rename the resulting file to '''addtabbeside. Another search:xpi'''
<blockquote></blockquote> The first result is not surprising, and we're back to the tabbrowser widget. The '''loadOneTab''' method calls another method to actually create and insert the new tab:  var tab = this.addTab(aURI, aReferrerURI, aCharset, aPostData, owner, aAllowThirdPartyFixup); Since '''addTab''' is You can try installing your extension in a method of ''browser that doesn'this''' we can search within the current document (CTRL+F) to find the '''addTab''' method. Finally we've found the right spot! <blockquote></blockquote>  this.mTabContainer.appendChild(t); Now all that we have to do is modify it to insert rather than append. =The by simply dragging 'How': the necessary changes to the code= There are different ways you could go about making this change, and someone with more experience using tabbrowser might recommend a different strategy or outcome. I decided to work on something that I knew nothing about in order to highlight the process one goes through, or at least the process I went through, when working with someone else's code. Since my goal is to show you how to do this, I also discuss my errors and mistakes below--they are an important part of the process tooaddtabbeside==First Attempt== The goal is to make as small a change as possible, since the existing code works well--I just want it to work slightly different. I'm also not interested in reading all of the code in order to make such a small change. I want to leverage as much of what is already there as I can. I assume that the xpi'''appendChild()''' method is responsible for the behaviour I don't like (i.e., adding new tabs to the end of the list). I'm not sure what to replace into it with, so I do another search inside tabbrowser.xml (i.e., using CTRL+F) looking for other methods/attributes of '''mTabContainer'''. I come-up with some interesting options:  index = this.mTabContainer.selectedIndex; ... this.mTabContainer.insertBefore(aTab, this.mTabContainer.childNodes.item(aIndex)); ... var position = this.mTabContainer.childNodes.lengthThis should trigger the add-1; I decide that I can probably accomplish my goal using these alone, and so start working on a solution. Here is my first attempt, showing the changes to '''mozilla/toolkit/content/widgets/tabbrowser.xml''' manager and give you the '''addTab''' method:  // Insert tab after current tab, not at end. if (this.mTabContainer.childNodes.length == 0) { this.mTabContainer.appendChild(t); } else { var currentTabIndex = this.mTabContainer.selectedIndex; this.mTabContainer.insertBefore(t, currentTabIndex + 1); } I then repackage the toolkit.jar file (change ''objdir'' option to install your objdir name):   $ cd mozilla/''objdir''/toolkit/content $ make then run extension and restart the browser to test (NOTE: ''minefield'' is my testing profile):  $ ../../dist/bin/firefox.exe -p minefield --no-remote I try to create a new tab using '''File > New Tab''' and nothing happens. ==Second Attempt== Clearly my code has some problems, since I've completely broken addTab. I decide to look for clues in the '''Error Console''' (Tools > Error Console) and notice the following exception whenever I try to add a new tab: <code>Error: uncaught exception: [Exception... "Could not convert JavaScript argument" nsresult: "0x80570009 (NS_ERROR_XPC_BAD_CONVERT_JS)" location: "JS frame :: chrome://global/content/bindings/tabbrowser.xml :: addTab :: line 1161" data: no]</code> I make a guess that childNodes.length is not zero, but 1 by default (i.e., there is always at least one tab, even if it isn't visible). A quick modification to the code, and I test again:  if (this.mTabContainer.childNodes.length '''== 1''') { ... ==Third Attempt== This works, but only the first time I create a new tab. Clearly I still have some misconceptions about how '''mTabContainer.selectedIndex''' and '''mTabContainer.insertBefore()''' really work. I can't yet see how my code is wrong, but the exception I'm getting clearly indicates that I've got some sort of type conversion problem. I decide to look again at the code examples in tabbrowser.xml that I'm using as a guide, specifically '''insertChild()'''. After a few seconds the error is obvious: I've used an Integer where a Tab was required. Here is the corrected code:  // Insert tab after current tab, not at end. if (this.mTabContainer.childNodes.length == 1) { this.mTabContainer.appendChild(t); } else { var currentTabIndex = this.mTabContainer.selectedIndex; this.mTabContainer.insertBefore(t, '''this.mTabContainer.childNodes.item(currentTabIndex + 1)'''); }
==Success, and some bugs==
After repackaging the toolkit.jar file and running the browser, I'm able The rewrite to confirm that this last change an extension has been successful. Opening a new tab now works in the way I originally describedsuccess. I make a few more tests In both cases we've managed to insure that I haven't broken anything else, for example, what happens if I am on achieve the last tab and not in the middlesame goal using almost completely different methods. This works, which makes me realize that using Using an extension we'''append()''' is probably not necessary at all, and I can safely shorten my code down ve made it possible to share our changes with any Firefox user worldwide without having to the following:ship a custom build.
// Insert tab after current tabHowever, as was the case in our previous attempt, not at endour code has a bug. var currentTabIndex = this.mTabContainer.selectedIndexMoving existing tabs doesn't update our position state, since we only modify '''mPreviousIndex''' when a new tab is selected; thismoved tabs remain selected, but change their order (i.mTabContainere.insertBefore(, TabSelect won't, thisget dispatched on a move).mTabContainer.childNodes.item(currentTabIndex + 1));
This means that six lines Luckily we've already stumbled upon the solution to this problem--the '''TabMove''' event. Here is the updated version of code become two'''addtabbeside.js''', and with that reduction in number of lines, hopefully a reduction the changes in new bugs I've added (NOTEbold: within reason, favour fewer rather than more lines of code).
Speaking of bugs, a closer read of '''addTab''' (see [http: var AddTabBeside = { //lxrState info on the last tab to be mPreviousIndex: 0, onLoad: function() { /seamonkey/sourceAdd a listener for the TabOpen event, which gets called as /toolkit/content/widgets/part of addTab in tabbrowser.xml#1219 line 1219] var container = gBrowser.tabContainer; container.addEventListener("TabOpen", this.onTabOpen, false) would indicate that ; // Also add a listener for TabSelect so we've introduced know when focus changes to a few with our new positioning code:tab container.addEventListener("TabSelect", this.onTabSelect, false); '''// wire up And a progress listener for the new browser object.TabMove to fix an edge case''' var position = '''container.addEventListener("TabMove", this.mTabContainer.childNodes.length-1onTabSelect, false);''' var tabListener = this // Finally, add a listener for shutdown window.mTabProgressListeneraddEventListener(t"unload", bthis.onUnload, blankfalse); }, onUnload: function() { // Remove our listeners var container = gBrowser.tabContainer; container.removeEventListener("TabOpen", this.onTabOpen, false); container.removeEventListener("TabSelect", this.mTabListeners[position] = tabListeneronTabSelect, false); '''container.removeEventListener("TabMove", this.mTabFilters[position] = filteronTabSelect, false);''' }, onTabSelect: function (e) { // When a different tab is selected, remember which one. This is // necessary because when a new tab is created, it will get pushed // to the end of the list, but we need to know where to put it. this.mPreviousIndex = gBrowser. ttabContainer._tPos = positionselectedIndex; }, Where the assumption before was that onTabOpen: function (e) { // Get the newly created tab was at , which will be last in the list var newTab =; // Move this new tab to the end right of the listpreviously selected tab, // checking to see how many tabs there are currently. By default // there is 1 tab, and the new code breaks thatfirst time onTabOpen is called, there will // be 2 (the default plus the newly created tab). ThereforeIn this case, we also need to update the value of '''position''don't // move the new tab, since it is already in the right spot. In all // wire up a progress listener for other cases, move the tab to the right of the new browser objectcurrent tab. if (gBrowser.tabContainer.childNodes.length > 2) { var position = currentTabIndex gBrowser.moveTabTo(newTab, this.mPreviousIndex + 1); } }, };
No other obvious defects are visible from // Insure that our changescode gets loaded at start-up window.addEventListener("load", function(e) { AddTabBeside.onLoad(e); }, false);
The change I was making was simple enough that I didnMany of the steps we did in order to create the extension't bother looking at any documentation or using s installation and registration files will be the JavaScript debuggersame each time. I found out afterward that tabbrowser has good Rather than doing it by hand, you can use [ documentation extensionwiz/ an on MDC-line wizard]. Another trick worth trying when you're making lots of JavaScript changes like this is to add the following line to your build a basic extension.mozconfig file:  ac_add_options --enable-chrome-format=flat
This will cause The process of developing extensions is greatly improved with the .jar files to be expanded so that you can edit the .xml/.js/.xul files in place and skip the repackaging step above (see addition of [ If you also use Setting_up_extension_development_environment some preferences] to the browser, as well as the [ Extension Developer's extension] . This extension will automate many of the steps described here. It will also allow you can to reload the browser's chrome at runtime. This is a great way to test changes to your XUL/JS files without restarting having to restart the browser.
You can read complete details on how to set-up an extension developer's environment [ here].
* [ Extensions on MDC]
* [ Extension Development on Mozillazine]
* [ XUL Event Propagation]

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