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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': create an Extension to change the way new tabs get created in Firefox=
As was [[Dive into Mozilla Modifying Firefox Lab#The_.27What.27:_change_the_way_new_tabs_get_created_in_Firefox|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': figuring out where to put our code=
The first thing we should address is why we're doing this at all: why bother creating an extension in order to do what we've already done in the tree? There are a number of answers to this question.
==Changes in the tree vs. an 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 https://bugzilla.mozilla.org and then creating and attaching a patch with your changes. The problem here is that even though you (and I) 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 be marked '''WONTFIX''', which means that your patch won't make it into the tree.
What does this leave? You could [http://en.wikipedia.org/wiki/Fork_(software_development) 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 users an extension to install, 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.
Having already successfully made this change once before, a logical first thought would be: "How can I modify my existing code to create an extension."
window.addEventListener("load", function(e) { AddTabBeside.onLoad(e); }, false);
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.
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 [http://ted.mielczarek.org/code/mozilla/extensiondev/index.html Extension Developer's extension] you can reload the chrome without restarting the browser.
=Resources=
* [http://developer.mozilla.org/en/docs/Extensions Extensions on MDC]
* [http://kb.mozillazine.org/Extension_development Extension Development on Mozillazine]
