You can check if there is more detail available about the issuer of the certificate. *Click the "Advanced" button show more detail *Click the blue error text (SEC_ERROR_UNKNOWN_ISSUER) to show the certificate chain *Click "Copy text to clipboard" and paste the base64 certificate chain text in a reply If clicking the blue error text doesn't provide the certificate chain then try these steps to inspect the certificate. *Open the Servers tab in the Certificate Manager **Options/Preferences -> Privacy & Security<br>Certificates: View Certificates -> Servers: "Add Exception" *Paste the URL of the website (https://xxx.xxx) in its 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.