Lightweight and Brilliant! Rated 5 out of 5 stars
I've been using Karma Blocker for years now and love it! It's amazing how lightweight it is (doesn't bog down FF at all). I've had some issues in the past with it blocking Google Maps and Facebook Ajax calls, but it's really easy to tweak the rules to "whitelist" certain domains. It looks intimidating at first, but once you look at other samples, you'll quickly learn how to work with it.