Error code: SEC_ERROR_REVOKED_CERTIFICATE

Firefox probably is the last browser that checks in real time whether a site's certificate has been revoked by its issuer (most browsers use a list that is updated from time to time). If you want to double-check the certificate, you can use this test page: https://www.ssllabs.com/ssltest/ It takes a couple minutes to check the site, so you might want to put that tab in the background while you do other things.