I actually don't use any speed-increasing add-ons (Firefox loads fast enough for me) but I personally recommend:
- AdBlock (laugh when all your friends complain about ads!)
- Download Statusbar (get rid of that annoying downloader pop-up!)
- THE BUILT-IN SPELLCHECKER (everyone needs this)
- if you use Gmail, Gmail Notifier
- and ForecastFox is nice (it's currently 18F here Q_Q)
EDIT: wait, you're not a new FF user, right
if you aren't, then woops