Agree with the re-install suggestion.
I got on fine with Firefox 3.5.5 and in fact found it a little faster in execution than IE8 (though slower than bonkers fast Google Chrome). I'm currently using Mozilla Namoroka 3.6 Alpha 1 (a version of the browser still under development) and find the same.
My only real "complaint" with both Firefox 3.5.5 and Namoroka 3.6 Alpha 1 is that they are much slower to initially start than both IE8 (which has the advantage I understand that parts of it are already running as a result of Explorer being part of the o/s) and Google Chrome. IE takes just over a second to start for the first time, Google Chrome not even the blink of an eye (it is instantaneous to all intents and purposes), but both Firefox 3.5.5 and Namoroka 3.6 Alpha 1 require just over six seconds.
The PC in question has a 2.4Ghz Intel Core Duo chip, 3GB RAM, XP MCE 2005, a separate HDD used only for o/s and apps, and is scrupulously maintained for performance.
Consistent and similar observations twixt Chrome and Firefox 3.5.5 on all three other (less powerful) computers in the household.