Have installed Firefox 3 beta on my Mac - not too shabby!
Only complaints so far (which are relatively minor) - the scribefire extension doesn't work yet, and my system hasn't yet totally adjusted yet.
Webpage rendering is much faster compared to firefox 2.
Seems pretty good so far. Considering that outside of gaming (at home) and excel sheets (at work), nearly everything I do is based out of a browser now*, it's good to see that things are developing quickly and working robustly.
The extensions on Firefox are also still pretty useful to me; I've started to use del.ico.us (it's blocked in China so I was late in ever trying it out) in addition to Foxmarks, scribefire, and download status bar.
*It's interesting noting really to what extent this is coming true. My company runs their official email application and groupware suite via a web-based client, and most of our internal systems are web-based as well. While I'm running a Vista laptop at present, to me there really isn't a compelling reason for why I'd ever need a windows OS at work (I guess Office for running Excel...that's really about it). At home, I'm running most of my personal stuff on my laptop, which runs OS X but 95% of the time is used for web browsing, which is most often done with Firefox. Anyway, I guess my point is that I have no compelling reason to use Vista, and given the performance hit it tends to require + the need to upgrade to significantly beefier hardware in order to use it for gaming, I do not see any reason to upgrade anytime soon.