Howie suggested FireBug as a must-have extension. While I agree it is amazing for debugging web pages, the reason I've had it uninstalled is what I'd call a bug. When you have it switched off for all pages, it still causes a very noticable lag when you open a new, blank tab. I'm not the only one either.
If this could be fixed, I'd probably keep it installed. As it is, I install it when I'm doing hardcore dev stuff, but keep it disabled most of the time.
In other Howie news, if you've seen Control, Howie lives just around the corner from where Ian and Deborah Curtis live in the film. How he justifies living in Sunny Macclesfield after living in Sydney or so long, I don't understand, but each to their own. If you haven't seen Control, go and see it. Best movie of the year.