A couple more about:config tweaks that I find useful:
Spellchecker covers forms too:
layout.spellcheckDefault = 2
Control key and mouse wheel scrolls one page instead of zooming:
mousewheel.withaltkey.action = 1
More items in recently closed tabs menu:
browser.sessionstore.max_tabs_undo = 15
Faster tab scrolling:
toolkit.scrollbox.scrollIncrement = 75