You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates. *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 and inspect the certificate and check who is the issuer of the certificate. You can see more details like the intermediate certificates that are used in the Details tab. Who is the issuer of the certificate? ----- If you can't inspect the certificate via "I Understand the Risks" then try this: Open this chrome URI by pasting or typing this URI in the location/address bar to open the "Add Security Exception" window and check the certificate: *chrome://pippki/content/exceptionDialog.xul In the location field type/paste the URL of the website *retrieve the certificate via the "Get certificate" button *click the "View..." button to inspect the certificate in the Certificate Viewer