Firefox 3, although currently it is Beta 4 (download), is good. Two things I like about it is that it uses less memory, and it actually allows you to see the result of your password before prompting to save it. Also has revamped bookmarking system and different icons - but these I don't really care about much.
The only drawback is absence of support for FF2 addons which effectively limit functionality.
Here are the 3 things I'm looking for in FF3 before I change from FF2:
1. Undo close tab
FF3 supposedly has this, but the recent tabs in History is disabled. This can be obtained in FF2 through Tab Mix Plus.
2. Open tabs to the right of current tab
True a very simple feature, but helps tremendously in organizing tabs. Again through Tab Mix Plus.
3. Del.icio.us addon
It is the one service which I rely on and which has yet not been upgraded to support FF3.
Hope these changes are done soon. Till then, I'll keep an eye on the compatibility of addons, and I'll stick to FF2.