StumbleUpon is awesome. More than just an add-on, it is actually a service. It is what some people call channel surfing for the internet. I would have to agree with that. With StumbleUpon (SU), you define a bunch of interests, based on these interests you can "stumble - upon" really cool sites.
The way they match you up to sites is pretty nifty I think because most of the time I'm finding out really cool stuff. You can even "tune in" to a specific channel and get nothing but websites related to that topic. Every time you get a website from SU you have the chance of saying "I like it!" or "no more like this". Using one of these two options gives the system a better idea of what you like and continues to narrow down topics that are more likely to appeal to you. You also get to see pages that your friends have voted as "I like it" or "thumbs up." SU also offers a bunch of other social networking tools. Blogs, messaging, groups are all part of the SU offerings, but its core is serving you new pages, fresh and original content is its forte. A great combination of your preferences, your voting for "thumbs up" or "thumbs down" and a good mix of your friends' own preferences continues to serve really cool pages time after time.
This is another extension or add-on that I must have in almost every installation of Firefox I use.