I only have 512MB of memory, run XP Home, and have had 3.5 since the day it was released - and both computer and browser run very fast.
This is a 2004 Dell Computer, so hardly state-of-the-art hardware.
Not sure why so many folks are having trouble with this version of FF hogging memory.
FF consistently uses around 75MB of memory (virtual memory is appx. 190MB).
Bookmark for later.