If you're anything like me with a ghetto box and running Firefox sometimes (usually with IE) this little tweak will help FF from holding too much of your precious ram. Of course if you got hordes of ram I wouldn't bother but you might want the ram saved for other sutff too? Now all this tweak does is take the memory FF uses for its windows already open rather than keeping it in memory is to put it to the HDD.
On to the tweak pinched from -
http://tech.cybernetnews.com/2006/03/26/this-may-help-your-firefox-memory-leak/ Open Firefox and go to the Address Bar. Type in about:config and then press Enter.
Right Click in the page and select New -> Boolean.
In the box that pops up enter config.trim_on_minimize. Press Enter.
Now select True and then press Enter.
Restart Firefox.
Works for you? Let me know how it goes :)