Credit:
The add-on, currently in beta, makes sure you connect securely to the following sites: DuckDuckGo, EFF, Facebook, Google Search, Google Services, Identica, Xquick, Mozilla, NYTimes, PayPal, Scroogle, Torproject, Twitter, The Washington Post, Wikipedia, GentooBugzilla and Noisebridge. Of course, you can make sure that you connect to these sites via HTTPS individually, but this script makes it a far simpler affair.
Installing this add-on is a good idea for users who often use public Wi-Fi hotspots; having your connection encrypted via HTTPS is the only thing that keeps you safe from various sniffing attacks.
If you're the tinkering type, you can create your own rule sets for the script; you can find out how to do that here.