I'm presently running Firefox 3.6.8 on Ubuntu 9.04. My Firefox extensions include Ubuntu Firefox Modifications 0.9rc2. I've upgraded Firefox on this install a few times; as far as I can recall, some version of this extension was present when I initially installed Ubuntu 9.04. I have disabled it, mostly because I didn't know what it provided me, nor was I able to find out when I tried a bit.
What does the extension do?
It came with xul-ext-ubufox 
:
Ubuntu-specific configuration defaults and apt support for Firefox.
Adds Ubuntu-specific modifications to Firefox.
Integrates the browser with Ubuntu to:
- Enable searching for missing plugins from Ubuntu software catalog
- Add the following options to the Help menu:
- Get help on-line
- Help translating Firefox
- Ubuntu Release Notes
- Set homepage to Ubuntu Start Page
- Display a restart notification after upgrading Firefox
- Add ask.com to the search engines.
You can uninstall this if you prefer to use a pristine Firefox install.
Homepage: https://launchpad.net/ubufox
The files here show some of the changes that Ubufox provides (custom branding, search preferences, etc.)
http://bazaar.launchpad.net/~mozillateam/ubufox/trunk/view/head:/defaults/preferences/ubuntu-mods.js
