I've still got Mandriva 2008.1 installed on europa, the aging 32-bit Athlon XP system, and I'm running Firefox 3.0.2 I had downloaded from Mozilla and installed (untarred) into my home directory. Up to this point I've not had any memory issues, but today I happened to look up at the system monitor running in my upper panel and saw that a good chunk of the 2GB of system memory was consumed along with 1GB of swap. So I opened system monitor, then exited Firefox and watched memory consumption. Sure enough nearly all swap was recovered as was nearly 1.75GB of system memory.
Normally my Mandriva box stays up all the time along with a single instance of Firefox with multiple tabs, and I don't usually have any problems. Firefox this time had 21 tabs open, which is not a lot for me.
The image to the right shows memory usage right before I exited Firefox. I let the system sit 'idle' for a few moments before I restarted Firefox again. You can see the corresponding network activity on the lower right, with a very modest increase in system memory consumption. Then it settles down. I have no real idea what caused it, but it would seem that there are still a few corner cases with regards to memory leaks within Firefox 3. I wonder if the developers have killed a few more memory leaks for 3.1 without introducing any new ones. I guess we'll see.