This add-on for Firefox will allow any user to know if anybody is trying to steal their social network access data. It's very important to know if the services that you use are secure, and if you check that they aren't always ask for encrypted connections to take place. Firesheep works if you're connected to an open public Wi-Fi network.
Once installed, a tab that will show the data of the users that are connected to that unsecure network will appear. By simply moving your mouse over the icon of each user and service, you'll be able to know the user name and password that they used when they accessed that social network.
Firesheep can come in handy to realize how dangerous it is to visit certain social networks on unsafe connections.