At the moment, I've also got Chrome and Opera installed. (Which leaves only Trident-based browsers, which I can't install without
ugly hacks.)
Strangely enough, my experience with Firefox has been very hit-or-miss. On my desktop at home (Ubuntu 10.04) and at work (Windows XP) it works great. On my laptop (Windows 7) it's every bit at bad as IE is. (Slow, freezes randomly, crashes, etc.) Actually, I think IE8 performs a bit better on my Laptop. (Not that I use it. I still need my extensions...)