remove those - http://kb.mozillazine.org/Problematic_extensions

http://www.ubuntugeek.com/speed-up-firefox-web-browser.html

http://ubuntuforums.org/showthread.php?t=1193567 ( however, using Swiftfox doesn't help at all, it make my browsing slower in fact )

Use tmpfs for profile - http://blog.taragana.com/index.php/archive/how-to-speed-up-firefox-significantly-with-tmpfs/

Use latest 64bit flash - http://www.adobe.com/software/flash/about/