Firefox 4, 5, and 6 were not good releases when it came to memory consumption. Firefox 7 had some big improvements. Firefox 8--14 all had smaller improvements

Firefox 4, 5, and 6 were not good releases when it came to memory consumption. Firefox 7 had some big improvements. Firefox 8--14 all had smaller improvements. Firefox 15 has this add-on leak prevention, which is another big win. And we're still working on this :)