I don't think FF 3.6 supported HTML5.
HTML5 introduces many things like a graphics engine, video rendering, and other features which FF have to adopt in order to stay alive in the browser wars.
FF 3.6 ran very well on my old-ish netbook, I've just updated to FF 25 from the slow memory intensive FF 21 and it appears to be a slight improvement.
I just wish FF would use slim line tabs or some other option that would not encroach on my screen's limited real estate.