Steps to repackage Mozilla Firefox:
The source is an EXE file.You need to repackage it to convert it into msi package using setup capture technique in Adminstudio.Prior to setup capture create three files as below.These files are used to suppress updates,check for default browser check and other configuration etc.. and lock the preferences so that user will not be able to edit them.
First create a file named mozilla.cfg with your custom configuration.
Example:

// disable default browser check
pref("browser.shell.checkDefaultBrowser", false);
// disable application updates
pref("app.update.enabled", false);
// Set default homepage - users can change

defaultPref("browser.startup.homepage", "http://home.example.com");

This file needs to be added to "programfiles(x86)\Mozilla Firefox\" folder inside your package.
Second create a file named local-settings.js and add the below contents.
pref("general.config.obscure_value", 0);
pref("general.config.filename", "mozilla.cfg");

More about locking preferences - http://kb.mozillazine.org/Locking_preferences
This file needs to be added to "programfiles(x86)\Mozilla Firefox\defaults\pref" folder inside your package.
Third create a file named override.ini and add the below lines.This file is used to disable the browser Import Wizard on first run.
[XRE]
EnableProfileMigrator=false
This file needs to be added to "programfiles(x86)\Mozilla Firefox\" folder inside your package.

Capture the package using setup capture technique in AdminStudio and create a mozilla.Irp file. Remove unwanted files and registries. Remove desktop shortcut.Build to create mozilla.Ism file.

Place the above said three files in their respective locations and bulid to create mozilla.msi file.
Validate for any errors.Once no error is found you can proceed with the testing part.