I used to work in tech support, so I have to ask...did you restart your computer? ;) It is probably some plugin that worked perfectly under 3.0 that is not pla

I used to work in tech support, so I have to ask...did you restart your computer? ;) It is probably some plugin that worked perfectly under 3.0 that is not playing well under 3.5. Try disabling all of your plugins and re-enabling them one by one. For me it was BetterGmail that was crashing FireFox. Disabling it fixed everything.