The only reason anyone on a wired LAN would be in danger of having their traffic sniffed is if there is a hub somewhere in the setup (highly unlikely these days) or a man in the middle attack is set up. Switched networks only send packets to the devices that need them, thus the sniffer tool Firesheep sees nothing. Man in the middle would require a computer(or hacked router, etc) with 2 NICs set up to route transparently, and would only impact any computers downstream.