Firesheep is an extension for the Firefox web browser [that] uses a packet sniffer to intercept unencrypted cookies from certain websites (such as Facebook and Twitter) as the cookies are transmitted over networks, exploiting session hijacking vulnerabilities.
It shows the discovered identities on a sidebar displayed in the browser, and allows the user to instantly take on the log-in credentials of the user by double-clicking on their name.
The extension was created as a proof of concept that many web sites only encrypt the login process and not the cookie created from it, and only posing a security risk that is exploited by the extension.