Should not matter - except you miss the little extra speed in 2. On a noticable and similar level as IE6 vs. IE7 to my eyes. Dont need benchmarks for proof. And a billion bugfixes of course. You could see problem fixed in 2. Strange things can happen, didnt Firefox use to have some scripting problem with SMF forum? Was going into loop trashing servers? You only see zzzzz on this SMF forum or does it go for phpbb, vB as well? Check SMF site if not. Many people should be screaming. Ive not tried 1.5 vs. SMF 1.1 so cant tell. Anyway, best way is to exclude. Clean profile will point finger at extensions, perhaps extension mix - or not.
Throw that line in user.js so all extensions will "work", dump new 2.0.1 zipfile into Firefox folder. Easy to upgrade. Cant really think of any reason not to. Most gadgets have been upgraded by now.