Adblock
One of the coolest features of Mozilla is the "Block Images from this Server" item in the context menu. It allows you to block annoying advertisements from web pages, but one of the more annoying things about it is that you have to configure it every time a new ad comes up, and it works only based on the server the ad comes from, so if you're up against an ad that is hosted by a server that also hosts legit pictures, you'll have to endure those ads. Plus it doesn't work against the far more annoying Flash ads.
Well, not anymore. I was going to make a feature request to the Mozilla team to include Flash blocking in a future version, but I found that many others already had. And, even better, that there is an add-on that accomplishes most of what I would like: AdBlock.
I'm not even going to go into a discussion here about the morals of blocking ads. Look, I know a lot of these sites rely on their ad revenue stream, and I hate to deprive them of that. But the bottom line is this: if the ads weren't so fucking annoying, I wouldn't be going through all this trouble to block 'em. So there.