I just installed the latest beta release of Firefox 3.0 onto my machine (beta4) and while the change log and release notes are all referring to plugged memory leaks and improved performance, initial tests on my XP machine show something else. Sure enough, it is a beta release and my testing is far from scientific, but still.
Loading up my start page and then switching to another one, gets the stable release 2.0.0.12 up to just over 52.5MB in memory usage. When I do exactly the same with 3.0 beta 4, it needs 62MB, which is more, no matter how you look at it. Some of my extensions don't work yet, but mostly I'm missing my snazzy Pimpzilla skin. Guess I'll be uninstalling beta 4 soon and wait for beta 5 or the final release to appear before switching over. It'll give the developers more time to iron things out and get the extensions up to date as well.
Note : it's quite possible that your mileage may vary. Go give it a try if you're feeling adventurous.