If you do web development of any kind and use Firefox, you either use Firebug on a daily basis, or you really need to learn about Firebug.
I have a handful of plugins for Firebug, but I just came across two that look immensely useful.
XRefresh will watch local files and reload a page in Firefox when they change. Sure, it’s not that much work to switch to Firefox and hit F5 (or Command+R for those of you who haven’t turned off the Mac’s special functions for the F-keys and find me turning up your sound whenever I try to reload a page during pair programming… or Control+R for those who don’t use Macs at all), but little savings can add up — especially if you don’t even have to leave your text editor / IDE at all!
CSS Usage sounds like a godsend. It’ll poke around a site and tell you what CSS rules are actually used. This could be immensely useful for going through an old site and pulling out old, unused CSS.
To be fair, I haven’t actually tried either of these plugins yet, but I expect both to get plenty of usage soon.