Firesheep is an add-on designed for the Mozilla Firefox browser. This add-on allows people to intercept unencrypted cookies from various websites (including Facebook and Twitter), effectively stealing login credentials. The user can log-in with the stolen credentials and become that user.
The use of Firesheep is a violation of computer security or wiretapping laws in some countries and considered a computer crime. However, Firefox does not block the extension from being downloaded and running. You can block Firesheep from intercepting your cookies by using an encrypted network connection. Using HTTPS, SSL, a VPN (virtual private network) connection, or Wireless network security (WEP/WPA) can accomplish this.