What's that mean? Look for it in the next beta release of Firefox 4! If you can't wait, grab a nightly build, but when 4.0 is released, HTTP Strict-Transport-Security will be built-in and turned on by default.
Though the feature's core functionality is there, work on HSTS is not completely finished. There are still a few tweaks I'd like to make, mainly providing a decent UI so people can add/remove HSTS state for servers themselves -- but none of this is necessary to be specification compliant. As landed, HSTS is the behind-the-scenes implementation that listens to the HTTP Strict-Transport-Security header and follows those instructions.
In case you don't feel like trolling through the IETF Internet Draft specification but you want to figure out how it works, here's a quick summary:
- Over an HTTPS connection, the server provides the Strict-Transport-Security header indicating it wants to be an HSTS host. It looks something like this:
Strict-Transport-Security: max-age=60000The header's presence indicates the server's desire to be an HSTS host, and the max-age states for how many seconds the browser should remember this.
- For an HSTS host (e.g., paypal.com), any subsequent requests assembled for an insecure connection to that host (http://paypal.com), will be rewritten to a secure request (https://paypal.com) before any network connection is opened.
- Optionally, the header can include a second includeSubdomains directive that tells the browser to additionally "upgrade" all subdomains of the serving host. That looks like this:
Strict-Transport-Security: max-age=60000; includeSubdomains
If Firefox knows your host is an HSTS one, it will automatically establish a secure connection to your server without even trying an insecure one. This way, if I am surfing the 'net in my favorite cafe and a hacker is playing MITM with paypal.com (intercepting http requests for paypal.com and then forwarding them on to the real site), either I'll thwart the attacker by getting an encrypted connection to paypal.com immediately, or the attack will be detected by HSTS and the connection won't work at all.
There are more details in the specification, like some further restrictions on cert errors and the like, but this is the general setup, and I believe a pretty nice feature to increase your site's security.