When a user connects to one of these hosts for the first time, the browser will know that it must use a secure connection., Keeler explained. If a network attacker prevents secure connections to the server, the browser will not attempt to connect over an insecure protocol, thus maintaining the users security.
Now in Firefox 17
The preload list used for this feature has been seeded with entries from a corresponding list for Google's Chrome browser, Keeler noted.
The new feature is now available in the current beta version of Firefox 17, so it's available for anyone to check out.