SimilarWeb for Firefox helps you find web sites you might like
What's most interesting about SimilarWeb is the way it works with Firefox. When you install the Firefox add-on, you'll see a new button in your navigation toolbar that turns a sidebar on and off. Every time you visit a new page, SimilarWeb will find pages that cover similar topics. You can adjust the settings to display large thumbnails, favicons only, or a a list of web sites by name and favicons (you can fit far more sites on a page in this view than you can with large thumbnails). If you don't don't like having the sidebar take over the left side of your browser window you can move it to the right side.
If you want to help make SimilarWeb's suggestions better, you can visit any web page and suggest a similar site either using the sidebar menu or by clicking the little arrow next to the toolbar button.
[via Firefox Facts]