StumbleUpon lets you "channelsurf" the best-reviewed sites on the web. It is a collaborative surfing tool for browsing, reviewing and sharing great sites with like-minded people. This helps you find interesting webpages you wouldn't think to search for.
Requires: Firefox: 0.8 - 1.0+
Installation:
Start Firefox, click the 'Open Folder' button opposite, drag the file 'stumbleupon-1.9995-fx+mz.xpi' into the main Firefox browser window and an installation prompt window should then appear, simply follow the onscreen instructions from here.