Same setup. Sidebar shows for me after selecting it from the View -> Sidebar menu, however it pops up with a message that says "Run --fix-permissions first." Not sure where I'm supposed to run this flag.
$ 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.
I have filevault turned on. Moving the binary out of my home folder (and adding a symbolic link) solved the problem.