Packaging Firefox 21 is similar to packaging previous version of Firefox.
There is 1 change I found from the version 20 application.
Trying to suppress the Import Wizard during the first launch using the override.ini was not working.
In version 21, the import Wizard can be suppressed by placing the override.ini in %programfiles%\Mozilla Firefox and %programfiles%\Mozilla Firefox\Browser.
http://mike.kaply.com/2013/05/13/more-major-changes-coming-in-firefox-21/
For silent installation, the /INI=<path to ini> switch can be used to configure installation sequence. Sample ini is available in previous version links.
Hope this helps.
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