The Firesheep plug-in was developed by security researchers to highlight how insecure public Wi-Fi networks can be. Mission accomplished. Unfortunately, the tool works quite well, and its public availability places a relatively powerful snooping tool that requires virtually no hacking skills or exceptional tech knowledge in the hands of anyone.
Free Wi-Fi hotspots are convenient, but come with increased security risks as well. Another Firefox plug-in called BlackSheep was developed as a Firesheep alarm. It won't secure your wireless data, and it won't prevent your information from being snooped by Firesheep per se, but it will alert you when Firesheep is in use on the network you're connected to so that you're aware.
Note: WinPcap must be installed on your PC before installing Blacksheep. The "Download Now" button takes you to the vendor's site, where you can install the latest version of Blacksheep directly into your Firefox browser.