There is a great extension for Firefox called StumbleUpon. It adds a toolbar to your browser that looks like this:

After you install the toolbar, you will be asked to register at StumbleUpon.com. To discover new web sites, just click the Stumble! button, and a new interesting web site will be loaded in the browser. If you like the site, click on the thumbs up; if not, click on the thumbs down.
It seems like the StumbleUpon extension might have some privacy issues. I don't use it in my main Firefox profile. Instead, I've created another Firefox profile just for StumbleUpon use and other Firefox extension testing.
I highly recommend trying StumbleUpon. It is a great way to discover intersting web sites based on your interests.
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