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 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. See also: *https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER *https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean *https://support.mozilla.org/en-US/kb/troubleshoot-time-errors-secure-websites *https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message