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

...in progress...


In the 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.

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 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': change the way new tabs get created in Firefox

As was previously the case, our goal is to modify Firefox so that new tabs are created (i.e. positioned) beside the current tab instead of being appended to the end of the list.

However, unlike last time where we modified tabbrowser.xml directly, this time we will overlay our changes onto the browser at runtime, and alleviate the need for any direct changes to the code. This is possible using extensions.

The 'Where': finding the right spot to make the changes

It's one thing to say you'd like to change the browser's behaviour, but quite another to actually do it. 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 isn't the same as impossible.

How do you begin? First, let's start at the top and find some UI notation we can search for in the code. In our case, we can focus on the various methods for creating a new tab:

  • CTRL+T
  • Right-Click an existing tab and select New Tab
  • File > New Tab

The second and third methods are useful, as they provide us with a unique string we can search for in the code. Before we can change anything, we have to search and read existing code in order to understand where to begin--this is the standard pattern for open source and Mozilla development.

Search 1 - finding a UI string

We're looking for a unique string--"New Tab"==, so we'll use LXR's Text Search feature. Here are the results you get when you search for "New Tab":


Lots of results, many of which point to comments in the code. However, the first result looks interesting:


Here we see 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)

Looking closely at tabbrowser.dtd we see that our English string, "New Tab", has the following entity:

<!ENTITY  newTab.label           "New Tab">

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're after.

Search 2 - finding an ENTITY

Repeating the search with the newTab.label ENTITY value instead of the "New Tab" string makes a big difference--we have many fewer hits:


Not surprisingly, the first result is the same DTD file (i.e., tabbrowser.dtd) we already found. The second result looks interesting, though:


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):

 <xul:menuitem label="&newTab.label;" accesskey="&newTab.accesskey;"

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:


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:


In this case, the tabbrowser widget has the onnewtab property set to another function, BrowserOpenTab(); (i.e., Firefox seems to handle tab creation in a non-standard way, providing its own method instead of using the default). Since we want to find the definition of this function, we search for "function BrowserOpenTab(", which returns two results:


Again, we're interested in Firefox (i.e., browser) instead of SeaMonkey (i.e., suite), so we skip to the second result:


This shows us that we need to be looking for yet another function, loadOneTab(). Another search:


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 a method of this we can search within the current document (CTRL+F) to find the addTab method. Finally we've found the right spot!


Now all that we have to do is modify it to insert rather than append.

The '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 too.

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 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 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.length-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 and the addTab method:

// Insert tab after current tab, not at end.
if (this.mTabContainer.childNodes.length == 0) { 
} else {
	var currentTabIndex = this.mTabContainer.selectedIndex;
 this.mTabContainer.insertBefore(t, currentTabIndex + 1);

I then repackage the toolkit.jar file (change objdir to your objdir name):

$ cd mozilla/objdir/toolkit/content
$ make

then run 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:

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]

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) {
} 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 to confirm that this last change has been successful. Opening a new tab now works in the way I originally described. I make a few more tests to insure that I haven't broken anything else, for example, what happens if I am on the last tab and not in the middle. This works, which makes me realize that using append() is probably not necessary at all, and I can safely shorten my code down to the following:

// Insert tab after current tab, not at end.
var currentTabIndex = this.mTabContainer.selectedIndex;
this.mTabContainer.insertBefore(t, this.mTabContainer.childNodes.item(currentTabIndex + 1));

This means that six lines of code become two, and with that reduction in number of lines, hopefully a reduction in new bugs I've added (NOTE: within reason, favour fewer rather than more lines of code).

Speaking of bugs, a closer read of addTab (see line 1219) would indicate that we've introduced a few with our new positioning code:

// wire up a progress listener for the new browser object.
var position = this.mTabContainer.childNodes.length-1;
var tabListener = this.mTabProgressListener(t, b, blank);
this.mTabListeners[position] = tabListener;
this.mTabFilters[position] = filter;
t._tPos = position;

Where the assumption before was that the newly created tab was at the end of the list, the new code breaks that. Therefore, we also need to update the value of position

// wire up a progress listener for the new browser object.
var position = currentTabIndex + 1

No other obvious defects are visible from our changes.


The change I was making was simple enough that I didn't bother looking at any documentation or using the JavaScript debugger. I found out afterward that tabbrowser has good documentation on MDC.

Another trick worth trying when you're making lots of JavaScript changes like this is to add the following line to your .mozconfig file:

ac_add_options --enable-chrome-format=flat

This will cause 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 http://www.mozilla.org/build/jar-packaging.html). If you also use the Extension Developer's extension you can reload the chrome without restarting the browser.