I can't believe I haven't installed this earlier!
It's called "Ad Block". It's an extension, which lets you right click on a banner (gif, flash) and block it, or you can set up wildcards to block off whole ad networks.
I wonder how this is going to affect advertising on GFY! If this gets huge, advertisers will have a big problem.
Have a look:
https://addons.update.mozilla.org/ex...i ndows&id=10