Firesheep is a Firefox plug in used by hackers that will packet sniff the network they are on (hence it's use on open Wifi spots like at Starbucks). It can can easily be used (trust me, I tested it, a script kiddie can do it) to pick up and steal an authenticated session from many popular sites (social sites, shopping sites, etc), thus if you are on an open wifi network and lets say browse to Amazon to go buy something, when you log in, they can capture your session and also log in as you and thus shop under your account. If you go to lets Say Facebook, they too can then access your account. If you log off your session, their session will also die but that is you actually clicking the logoff option on the site.
There isn't much you can do about it and yes it is doing packet sniffing of the network so doesn't matter what browser you use, it is an issue with just how web traffic is and sites and sessions.
Same rules apply as they always do, when using a publicly open network, DON'T sign into anything.
