What does FF3 give you? Not much, IMHO, except some little (or not so little) annoying glitches:
I changed over to 3 on one computer several weeks ago, and will NOT do so on the other - unless they get that location bar issue sorted out, and the download history which is deleted every time I delete my private data, etc.etc. Much less ways to tweak FF the way YOU want it. Lots of community complaints and Bugzilla entries along the lines of "if I wanted MS IE I would have got MS IE".
I sincerely hope that with 3.1 they get it right - I was a FF2 fan.