Firesheep danger, protect yourself
At http://techcrunch.com/2010/10/25/firesheep/, two Firefox plugins are recommended. They force your Firefox to use the https protocol wherever possible.
I installed both. HTTPS Everywhere contains a list of predefined sites but it’s difficult to add a new site. Force-TLS is the opposite. So the two nicely complement one another :)
Update (20101109): you can find some more info here.