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Public Key Pinning helps ensure that people are connecting to the sites they intend. It allows site operators to specify who can issue valid certificates for them, rather than accepting any one of the hundreds of built-in root certificates that ship with your browser. If a site makes use of key pinning, and your browser sees a certificate chain for that site which does not match the pin, Firefox will reject the connection and display an error page.
The error page displayed by Firefox when you encounter a pin violation gives you the option of reporting this error. Doing this can help us to make the web safer.