I don't run Firefox very often, but I have it set to use the Scribblies theme to match the maturity of the underlying software (and why is it that 15 years on, there still aren't any Web browsers that don't suck?)
Under FF 1.5.x I run the Brushed theme... makes me feel more like I'm on a mac, even though it's not even close. Currently, though, I've been running FF2 Beta 2 with the default theme (as the brushed theme hasn't been ported).
My Thoughts on Beta 2:
I think it's quicker than 1.5.
It has built-in RSS, which, when I use it, it's nice.
A lot of my extensions have been rendered useless as the features have been built in (Move close to actual Tab, spell check for those unfortunate enough to be on Windows)
It's pretty stable... it's only crashed once and that was due to playing a WMV in the browser, so I'm not 100% certain that FF was at fault.