Also curious how many tabs are open at any given time. I keep it to two or three (at most) and shut down the browser twice a day (morning and afternoon/early evening). Clears memory in use, also allows FF and plugin updates (only use two NoScript and a video thingie) to install, if found. Do add at least another 512mb, though better to go 2gb if your motherboard supports it... don't know how old your hardware is.
One other "fix" is to uninstall FF (export your bookmarks to a different location first) then manually delete the FF folder(s) in all locations (on Win machines that will be under a user account and program files, make sure it is gone from registry as well) then reboot. Then install new. Done that for a few clients with similar problems, even on low memory machines, and that restored operations.