"Our preload list has been seeded with entries from Chrome’s list of a similar function," wrote Mozilla's David Keeler in a blog post. "To build our preload list, a request is sent to every host. Only if a host responds with a valid HSTS header with an appropriately large max-age value (currently 10886400, which is eighteen weeks) do we include it in our list. We also see if the includeSubdomains value for the entry on Chrome’s list is the same as what we receive in the response header (if they do not match, we use the one we receive)."
The approach is designed to mitigate a potential vulnerability that would allow an attacker to prevent a browser from securely connecting to a site via HSTS. With forced HSTS, the browser will never connect to an included website via an insecure (HTTP) protocol.
Users of Google Chrome can go a step further and control individual sites via the interface at chrome://net-internals/#hsts, which enables users to add or delete HSTS websites.
The current Firefox Beta can be downloaded here.