I believe most of the memory leak is fixed. However, doing a quick search on google I found this:
How to fix the Firefox memory leak (Firefox hack) // Internet Duct Tape
I haven't tried it myself, so I accept no responsibility that may go wrong when trying this. If possible, try on a test machine first. The comments are good, however. I'm going to try it soon.
EDIT: Interesting read:
Firefox 3 Memory Usage pavlov.net
EDIT #2: I have just tried the above procedure:
How to fix the Firefox memory leak (Firefox hack) // Internet Duct Tape
It really does seem to work. Memory usage has dropped dramatically. I especially like that it now uses less memory when minimized. BTW, I'm using WinXP SP2.