When using Firesheep on MacOSX you get:
“Couldn’t get netmask for device en1: en1: no IPv4 address assigned.”
Open up terminal and type “ifconfig” to see if your ethernet cart is indeed on en1. My Mac Air has the wireless card as en0, not en1.
Go to the url “about:config” in Firefox. This opens up extra configuration options for Firefox. Search for ‘firesheep’ and there will be an option to change the Firesheep interface.