The firefox extensions that you must have:

Adblock (block the unwanted image/flash, based on the wildcard filter)
Mouse Gesture (all-in-one, or other simple one)
Bookmark Sync (very useful, if you surf the web on different machine)
Linkification (make every URL clickable)

The firefox themes worth of checking:
littlefox (if you have small screen size, I mean less than 1024x768)
noia (very beautiful)

Check them out in firefox website. Don't be lazy, search it by yourself. Or right click the selected text and "Search the web ...", in firefox.