Firefox now supports built-in public key pins, which means that a shortened list of acceptable certificate authorities (CAs) for participating sites is built into Firefox. In this first stage of pinning roll-out, protected domains include addons.mozilla.org and Twitter, to be followed by Google and other sites in upcoming versions of Firefox. That means that Firefox users will be even safer when visiting Mozilla and Twitter (and soon, Google). For the full list of pinned domains and rollout status, please see the Public Key Pinning wiki. Additionally, sites may advertise their support for pinning with the Public Key Pinning Extension for HTTP, which we plan to implement soon.
Public Key Pinning helps ensure that people are connecting to the sites they intend. It allows site operators to specify which CAs issue valid certificates for them, rather than accepting any one of the hundreds of built-in root certificates that ship with Firefox. If any certificate in the verified certificate chain corresponds to one of the known good (pinned) certificates, Firefox displays the lock icon as normal. When the root cert for a pinned site does not match one of the known good CAs, Firefox will reject the connection with a pinning error. This type of error can also occur if a CA mis-issues a certificate. In this way, key pinning can be used by sites to add another layer of trust to their servers’ deployment of TLS.
For more details on how Pinning works, check out Monica’s blog post