This article provides helpful information when you want to create a custom version of Firefox.
Including extensions with your distribution of Firefox
Starting with Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), you can easily bundle extensions with your customized Firefox distribution that automatically get installed the first time a user starts up your distribution. This is done each time the user starts up an updated version of your Firefox distribution, unless they've either uninstalled the extension or have a newer version of it already installed.
To do this, simply place the extensions in the
distribution/extensions directory in the application's distribution directory.
You can get a list of the extensions installed this way by looking for preferences whose names are "extensions.installedDistroAddon." followed by the extension's ID.