One of the big complaints about Firefox amongst hardcore nerds has been the memory leaking issue (despite frequent rebuttal that it is by design, or whatever). The Mozilla guys have made this a big focus of Firefox 3 and the new beta 4 shows the improvements mentioned in this post by Pavlov, one of the developers. If you want to test beta 4 without blowing away or risking your current Firefox installation check out the portableapps.com build, which will let you run it independently.
It is noticeably faster for me and definitely is looking nice. Not sure if I’m keen on the new address bar, but I’ll give it a go.