Not a bug, feature. It caches pages recently gone to so when you go back, you can go back instantly.
If you want to have something that helps:
1) enter "about:config" in the address bar and go
2) create a new boolean entry, name it "config.trim_on_minimize"
3) set created entry to true
4)
5) not finishing this lame joke
But now if you minimize firefox, it'll empty out all of the memory it's using in this feature =)