AdBlock Plus - stop displaying any URL with wildcards. So if you don't already have, for example, doubleclick.net blocked, you can right click on any ad from them and go 'don't show me any more crap from them'. Fab, and faster than setting them to be unreachable.
Session Manager - save the open tabs, so you can reopen them easily. Remembers the lot if you have a crash or just switch off the PC too.
What do people recommend:
superT - if you have more than a couple of tabs open (it's not unknown for me to have hundreds), you need an extension to cope. This doesn't do as much as some (Tab Mix Plus or the 'everything plus the kitchen sink' of my old favourite, but doesn't work with Firefox 2.0.
So I'm looking for something that will handle lots of tabs well and not try to do too much.
Greasemonkey - alter web pages you're viewing. The page wants you to do two clicks when one should do? A short script fixes it. AdBlock is an easier to stop seeing things you don't want to see, but this is far more flexible.
Platypus makes this even easier - you can edit the webpage you're looking at and it'll generate the Greasemonkey script.