At the Toorcon 12 security conference, Eric Butler released a Firefox plugin named Firesheep, which drew significant media attention. Firesheep allowed any user to seamlessly hijack the web session of another user on the same local network. Although such attacks are not new, the ease of use presented by Firesheep brought session hijacking to the masses.
BlackSheep, also a Firefox plugin is designed to combat Firesheep. BlackSheep does this by dropping ‘fake’ session ID information on the wire and then monitors traffic to see if it has been hijacked. While Firesheep is largely passive, once it identifies session information for a targeted domain, it then makes a subsequent request to that same domain, using the hijacked session information in order to obtain the name of the hijacked user along with an image of the person, if available. It is this request that BlackSheep identifies in order to detect the presence of Firesheep on the network. When identified, the user will be receive the following warning message:
It should be noted that Firesheep and BlackSheep cannot be installed on the same Firefox instance as they share much of the same code base. If you want to run both Firesheep and BlackSheep on the same machine, they should be installed in separate Firefox profiles.

Zscaler Research: BlackSheep - A Tool to Detect Firesheep
Zscaler Cloud Security : SaaS Web Security, Web Security, URL Filtering, Internet Security

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YouTube - BlackSheep, a tool to detect Firesheep