Could you please provide more detail about the issuer of the certificate? *click the "Advanced" button show more detail *click the blue SEC_ERROR_UNKNOWN_ISSUER message to show the certificate chain *click "Copy text to clipboard" and paste the base64 certificate chain text in a reply If that didn't work then try these steps to inspect the certificate. *open the Server tab in the Certificate Manager **Options/Preferences -> Privacy & Security -> Certificates: View Certificates -> Servers: "Add Exception" *paste the URL of the website (https://xxx.xxx) in it's location field. Let Firefox retrieve the certificate -> "Get Certificate" *click the "View" button and inspect the certificate You can see detail like the issuer of the certificate and intermediate certificates in the Details tab.