Collusion is a very interesting add-on for Firefox, if you are interested in seeing how your data is tracked. Privacy fans will love this extension because it allows you to see third-party cookies and how they track your movement as you browse from website to website.
Here is a little more information about the extension from the developers:
If you don’t see the Collusion icon in the bottom right corner of your browser, make sure the add-on bar is shown. On Windows, click the Firefox menu button, then click Options, then check “Add-on Bar”. On Mac, go to the View menu, then the Toolbars menu, then check “Add-on Bar”. We’re working on adding more features, such as being able to upload an anonymized version of your Collusion data so we can build a big picture view of the tracker ecosystem. We’re also working on visualizing other methods of tracking besides third-party cookies and improving the visualizations generally.
Collusion is an experimental add-on for Firefox that allows you to see which sites are using third-party cookies to track your movements across the Web. It shows, in real time, how that data creates a spider-web of interaction between companies and other trackers.
Want to learn more? You can pick up Collusion from the Firefox Add-ons website.