I tried this myself and it works like a charm. Note, it is a reconfiguration of the default settings so if youre a beginner it may not be in your best interest to do this and youre ending up asking TT how to get it back :lol:
http://www.pctipsbox.com/how-to-double-firefox-speed/
http://www.pctipsbox.com/how-to-double-firefox-speed/