This is incredibly scary but you should be aware that any time you're on a public wifi it's probably not secure.
It’s not hard to comprehend the far-reaching ramifications of this tool. Anytime you’re using an open Wi-Fi connection, anyone can swiftly access some of your most private, personal information and correspondence (i.e. direct messages, Facebook mail/chat)— at the click of a button. And you will have no idea.
This is how it works. If a site is not secure, it keeps track of you through a cookie (more formally referenced as a session) which contains identifying information for that website. The tool effectively grabs these cookies and lets you masquerade as the user.
Apparently many social network sites are not secured, beyond the big two, Foursquare, Gowalla are also vulnerable. Moreover, to give you a sense of Firesheep’s scope, the extension is built to identify cookies from Amazon.com, Basecamp, bit.ly, Cisco, CNET, Dropbox, Enom, Evernote, Facebook, Flickr, Github, Google, HackerNews, Harvest, Windows Live, NY Times, Pivotal Tracker, Slicehost, tumblr, Twitter, WordPress, Yahoo, Yelp. And that’s just the default setting— anyone can write their own plugins, according to the post.
More on Firesheep @techcrunch
How to protect yourself in Firefox (involves the extension Force TLS) which will encrypt your data