Firesheep makes http hijacking real easy. This type of attack is not new, principle concept had been used many times in previous attacks. Firesheep makes the exploit easier as it is a point & click method, thats where the difference lies. https sites are immune to Firesheep attacks.
Firesheep looks for authentication cookies in open wireless networks & steals them. People using social network sites need to be cautious about their network.
This type of attack can be made by using tools like
wireshark but then it requires certain level of expertise.