Adblock - which allows you to block adverts, pop-ups and other annoying elements and scripts on any page. Very easy to use and makes a huge positive difference to browsing.
SessionSaver - so that you can quit Firefox and have all your open tabs reload on the next start-up. It also allows you to re-open recently closed tabs, which is handy if you close one by mistake.
I’ve been using others too:
Tabbrowser preferences - which adds more options to do with the browser tabs, unsurprisingly
Greasemonkey - which I’ve not had a lot of time to investigate, but has provided a couple of useful scripts to change the way some of my more frequently visited sites behave.