Wikipedia
Firesheep is an extension for the Firefox web browser that uses a packet sniffer to intercept unencrypted cookies from websites such as Facebook and Twitter. As cookies are transmitted over networks, packet sniffing is used to discover 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 the victim's name.
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. It has been warned that the use of the extension to capture login details without permission would violate wiretapping laws and/or computer security laws in some countries. Despite the security threat surrounding Firesheep, representatives for Mozilla Add-ons have stated 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 to test the security of one's own systems). Note that even if they did, it wouldn't actually prevent anyone from using Firesheep, as Firefox contains a setting to disable this blacklist.
Later a similar tool called Faceniff was released for Android mobile phones.