Some firewalls monitor secure (https) connections and send their own certificate instead of the website's certificate. * Click the link at the bottom of the error page: "I Understand the Risks" Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate". * Click the "View..." button to inspect the certificate and check who is the issuer. Only leave the mark in the box at the bottom to "Permanently store this exception" if you trust that certificate. * Click "Confirm Security Exception" to enter the site if you still want to go to that website. If the issuer is your security software then you either have to disable that check in your firewall or install the root certificate of that firewall. See [[Firewalls]]