I'm now using the newly released Beta 3 of Firefox and it seems to be doing well. One of the gains reported is better memory management and that seems to be working. I haven't done a comparative test, but a quick look at test manager tells me that the seven tabs I have open are consuming 85 Mb of memory and that is much less than was the case with earlier versions.
There's a long (four-page) article about the new release on the Mozilla.org site - it has the curious title of "Firefox 3 Beta 4 The Review" - so I imagine that there'll be another beta release before too long :-)
Another article on ZD Net tells us that the main gain in Beta 4 is on speed - Firefox runs five times faster than IE7