Firefox is known best for it's extensive amount of add-ons so i'll post my favourites, if this thing had a comments system i'd ask you to post yours too.

Fasterfox - very good addon that speeds up firefox. https://addons.mozilla.org/en-US/firefox/addon/1269

Adblock Plus - fantastic ad-blocking extension, blocks almost every ad. https://addons.mozilla.org/en-US/firefox/addon/1865

Down Them All - the best download manager i've ever used, increases speed, tons of features.
https://addons.mozilla.org/en-US/firefox/addon/201


User Agent Switcher - allows you to pretend to be GoogleBot and access some sites you may not normally be allowed in. https://addons.mozilla.org/en-US/firefox/addon/59

Vista aero - My favourite them for firefox, that makes it look just like IE7. https://addons.mozilla.org/en-US/firefox/addon/4988