My recent upgrade to Hardy Heron installed Firefox 3. I tried to use it, I really did, but I couldn't use it. First, it's butt ugly. It doesn't follow my widget theme at all, it uses the clunky looking default gtk widget look. That wasn't the real problem though. My main issue is the extensions don't work. Part of my job includes web development, so I run firebug, web developer, and some other extensions all the time.
1. Open synaptic, and mark remove completely for all the firefox 3 items you can find.
2. Apply the changes.
3. Install firefox 2.
4. Go into ~/.mozilla/firefox/weirdprofilename/extensions and remove all files.
5. Go into ~/.mozilla/firefox/weirdprofilename and remove extension.*
6. Start firefox, go to Tools - Add ons, and reinstall your plugins.
I have Firebug, Molybdenum, Measure It, Remove cookies for site, Selenium IDE, and Web Developer.