This thread was timely as I had noticed that FF was using up usually 120mb and up to 160mb of memory (no CPU issues for me). Now I understand when I open many tabs, but when I close them the memory usage doesn't drop.
So I uninstalled unused extensions and disabled ForecasFox but this didn't seem to help.
Then I found this which did the trick -
http://www.freerepublic.com/focus/f-.../1327586/posts