Hoople - 2:00 pm on Jun 22, 2011 (gmt 0)
I usually run with 5-8 tabs open. Ubuntu Linux 11.04. I usually restart FF 1-2 times a day to address the memory usage. I never leave FF open overnight as it's sure to swell to taking nearly all of available memory.
For me FF 3.5x was a memory pig that FF 3.6x addressed. FF4 used more memory than 3.6x but wasn't as bad as FF 3.5x. FF 5 for me seems about what FF 3.5 was memory wise.
My only concern thusfar is some extensions/plug-ins aren't working - notably the Google Toolbar.