Firesheep is a new security exploit (“hacking”) tool that allows wanna-be hackers with very little expertise to gain access to the accounts of their victims on a range of websites, including facebook, flickr, and twitter, when using an unsecured wifi network.
It’s particularly bad news for travellers, who tend to use unsecured wifi networks from their laptops, netbooks and smartphones — in cafes, guesthouses, etc — much more than the broader public.
Although the technique that Firesheep uses is not new, it was previously moderately difficult to perform. The release of Firesheep means that someone with almost no technical knowledge can gain access to other peoples accounts when using the same unsecured wifi network.
You will potentially be at risk if:
Luckily Firesheep does not allow the hacker to get hold of your password.

Even if you don’t use Firefox normally, it would be a good idea to use Firefox with the HTTP Everywhere extension whenever you’re accessing the internet via unsecured wifi. At least until the exploit is fixed.
More information:
How to protect against Firesheep attacks