I got Firefox 3.6 in a Lucid alpha update on 27 January. The problem is that it wouldn’t start. I tried it with a new profile, which made it work. So I ran firefox with the -safe-mode argument and disabled all add-ons. Then in firefox, I tried each add-on to figure out which one’s weren’t working, and left them disabled.
To do this, open up a console and run the following:
firefox -safe-mode
You’ll see the following dialog box:

Select the first checkbox (Disable all add-ons), and press the Make Changes and Restart button.
When Firefox starts up, go into the Tools->Add-ons menu. And try enabling each add-on and restarting firefox. If it starts, then the add-on is working. If it isn’t, run firefox in safe-mode and disable the last add-on you enabled.
In my case, Adblock Plus, Better Gmail 2, Better GReader, Firebug, FoxyProxy, Ghostery, HttpFox, NoScript, Nuke Anything Enhanced, and User Agent Switcher were working. DownThemAll, Greasemonkey, and Stylish were not.

click image to enlarge.