The first big post-FF4 improvements were in FF7, which was released in September last year. See http://blog.mozilla.org/nnethercote/2011/08/09/firefox-7-is-lea

The first big post-FF4 improvements were in FF7, which was released in September last year. See http://blog.mozilla.org/nnethercote/2011/08/09/firefox-7-is-lean-and-fast-2/ http://blog.mozilla.org/nnethercote/2011/08/09/firefox-7-is-... for some details. And things have improved quite a bit more since FF7.