Have you tried Firefox 3 yet?
If you're a Firefox user, and you haven't tried out Firefox 3 yet, you're missing out on some genuinely awesome features. I've been testing it since the early beta bits were available, but I think that the recently released RC2 build is stable enough for anyone to use.
If you're still hesitant, watch this screencast by Mozilla's Mike Beltzner, in which he introduces some of the major new features; you're bound to be impressed.
If I had to pick my favorite feature, it would have to be the revamped address bar (check out the screencast to see it in action), but I also like the integrated add-on manager, and pause/resume support in the new download manager.
Vista users - install the Glasser extension to enable the Aero Glass effect on the browser chrome. Now you've got a browser that fits in much better, visually, with the rest of the system. :)