There is so many hoops I have to jump to make this work in OS X.
$ mv firesheep-backend firesheep-backend.binary
$ cat > firesheep-backend
#!/bin/sh
sudo /path/to/firesheep-backend.binary $@
^D
$ sudo chmod +x firesheep-backend
Then restart Firefox and start capture. You need to run sudo once every certain period.