Firesheep was an extension for the Firefox web browser that used a packet sniffer to intercept unencrypted session cookies from websites such as Facebook and Twitter.

The plugin eavesdropped on Wi-Fi communications, listening for session cookies.

[2] The extension was released October 2010 as a demonstration of the security risk of session hijacking vulnerabilities to users of web sites that only encrypt the login process and not the cookie(s) created during the login process.

Despite the security threat surrounding Firesheep, representatives for Mozilla Add-ons stated initially that it would not use the browser's internal add-on blacklist to disable use of Firesheep, as the blacklist has only been used to disable spyware or add-ons which inadvertently create security vulnerabilities, as opposed to attack tools (which may legitimately be used in legitimate penetration tests).

A similar tool called Faceniff was released for Android mobile phones.