Image:

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 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 clicking the SEC_ERROR_UNKNOWN_ISSUER text doesn't provide the certificate chain 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.

cor-el said

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 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 clicking the SEC_ERROR_UNKNOWN_ISSUER text doesn't provide the certificate chain 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.

Ok, here is the info:

https://support.mozilla.org/1/firefox/59.0.3/WINNT/en-US/security-error Peer’s Certificate issuer is not recognized. HTTP Strict Transport Security: false HTTP Public Key Pinning: false Certificate chain: -----BEGIN CERTIFICATE----- MIIESTCCAzGgAwIBAgIQWvc8MiykIUKvH8S/AsI/0zANBgkqhkiG9w0BAQsFADCB gTE6MDgGA1UECwwxZ2VuZXJhdGVkIGJ5IEF2YXN0IEFudGl2aXJ1cyBmb3IgU1NM L1RMUyBzY2FubmluZzEeMBwGA1UECgwVQXZhc3QgV2ViL01haWwgU2hpZWxkMSMw IQYDVQQDDBpBdmFzdCBXZWIvTWFpbCBTaGllbGQgUm9vdDAeFw0xNzA5MjAwMDAw MDBaFw0xODEwMzExMjAwMDBaMIGGMQswCQYDVQQGEwJVUzETMBEGA1UECBMKQ2Fs aWZvcm5pYTEWMBQGA1UEBxMNTW91bnRhaW4gVmlldzEbMBkGA1UEChMSTW96aWxs YSBGb3VuZGF0aW9uMQ8wDQYDVQQLEwZXZWJPcHMxHDAaBgNVBAMTE3N1cHBvcnQu bW96aWxsYS5vcmcwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQC6Pc/5 Ym92iVxZe7zLOR7IB9wjZXc2oryl0KLp19QBajDP7r3X+UZlXP1ZC3h4N3CRKe6G OV4hqFmJe88FScwcKNGh1z42gXgsigOy87UDxAYRrvpjIinJz7oITeVZoIsf2UIm xdZ7dJyjhFi4rBj5WHNvtZPTwXvbpoax5/WGzAh1L6tOF1zABH3St5iHH+7XAiYI fC1SaR1fALUP+LUMROpkm9YImU40XO5f/zGtvFJjNtw11Y25ANN/m9GfLQu8hSN0 sW7ctT7P/NNyFQVb2Jme68axpP1eVEVSUD/EFx21sX8GOfiSCR5ibXTa+Rkh2NnV qEx1b3iA8KgXtbbNAgMBAAGjgbUwgbIwHwYDVR0jBBgwFoAUsjA21ZrvD+N4fN79 82thAsDscvwwHQYDVR0OBBYEFKNcAPirCasC5wR4TwpVz9cQH1ReMDMGA1UdEQQs MCqCE3N1cHBvcnQubW96aWxsYS5vcmeCE3N1cHBvcnQubW96aWxsYS5jb20wDgYD VR0PAQH/BAQDAgWgMB0GA1UdJQQWMBQGCCsGAQUFBwMBBggrBgEFBQcDAjAMBgNV HRMBAf8EAjAAMA0GCSqGSIb3DQEBCwUAA4IBAQCqAELV4JZ89KUbuMSgT5ESWyZB ma+bPgPmgK9j+6vzK7ftA7OfgTsHisyzkDR0BCjJKtqScD2Ibu+ozpnpEXqaqvTi UviCz0vBJtKfKV0jBOpobqnfnph7Fu6JR8P2+wdU5RaBpP5s/d+52uhSO8Cg3YCk YC9IFogQvQVC9Ak6u33zoBF9xbUuU39H1ikc9HW/2cLaP2h15q0WCXczgacr7dH5 GQyTRTSX5Pf3i7+qNqpBX8aNQLe2BM7G1Z8HE3vyv5QzaDrULb53vbkbqiZ56+0D 0db4RCUNf2zjaTZxrCo4HOJx0eHzyuD4dzL7dJQ3hxpBW3P1hHfEdvcRABIg -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- MIIEAzCCAuugAwIBAgIQai7m5TjgPUy0wRLuipZiRzANBgkqhkiG9w0BAQsFADCB gTE6MDgGA1UECwwxZ2VuZXJhdGVkIGJ5IEF2YXN0IEFudGl2aXJ1cyBmb3IgU1NM L1RMUyBzY2FubmluZzEeMBwGA1UECgwVQXZhc3QgV2ViL01haWwgU2hpZWxkMSMw IQYDVQQDDBpBdmFzdCBXZWIvTWFpbCBTaGllbGQgUm9vdDAeFw0xMDAxMDExMjAw MDBaFw00MDAxMDExMjAwMDBaMIGBMTowOAYDVQQLDDFnZW5lcmF0ZWQgYnkgQXZh c3QgQW50aXZpcnVzIGZvciBTU0wvVExTIHNjYW5uaW5nMR4wHAYDVQQKDBVBdmFz dCBXZWIvTWFpbCBTaGllbGQxIzAhBgNVBAMMGkF2YXN0IFdlYi9NYWlsIFNoaWVs ZCBSb290MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAu/9u2c1QaV4H DfaOeYmtio9gsIG1vH0KQLLRJIKEhMIO21+KzVNLadU0ksB9WqX0UdmxghgI1BUK gvHzbDh7X6jmRcUzU1pM5iifY20twF8bgRuvuuQCcsYoFFo4J/PVT5W87ilK8a9D h//YFZ28k9MpLWni/++oQa5M+MaXCc1lKZ4ZpJNvl5fJyIftF5Mtv8vNc1Dx8JfZ WsnOzmwcOIlWJpj3FTT+gQqgSh7W+CsEapPOCKOedKVVUYXXwuwykSrIGzRd8Qmh E0ouGZdsi51S3DnVG7NX+34xWvVjtpo6IzG2TDTA9e9i5AmUIjBC9T0qdLAEBiWF kWMfciSCKQIDAQABo3UwczAPBgNVHRMECDAGAQH/AgEAMAsGA1UdDwQEAwICBDAT BgNVHSUEDDAKBggrBgEFBQcDATAdBgNVHQ4EFgQUsjA21ZrvD+N4fN7982thAsDs cvwwHwYDVR0jBBgwFoAUsjA21ZrvD+N4fN7982thAsDscvwwDQYJKoZIhvcNAQEL BQADggEBAI7N5ptR6zHHytYbmRcTcxVP3lwAQE5BMQ+aEanOBUP6RAQLnEGudZiY 5ch8E2jENqahvN9Y1XRFXRdS7JC8RLM0DZGj2h2SXrIcXK/HRMInPdoqoxdviULk uRPeAuDYM+M4UTzD+dWhdtf0sIHig4uezL+Hm2T15uz+Xd0zG0dY9ADDzAT48xDN YsscpjabtyFGOQQ4nC4yiqGWjoAGC+sDlZCxlN5ZJ6uMCYV7Yrhq4yl7XBjczYeo PgYEKrbvIMBcCmmg7wqm6oWjwqYLb0OSir//ZaYwb91evQQUnxf3Baz9zxvnJ7Vs BN3Ue1uN2Lvm7Io4D+JBPCPxfoCgiQs= -----END CERTIFICATE-----

I found this from where you said also-this leads me to believe it might be something to do with my Antivirus (Avast):

Yes, the issuer of the certificate is your security software.

Subject 	C=US, ST=California, L=Mountain View, O=Mozilla Foundation, OU=WebOps, CN=support.mozilla.org
Issuer 	OU=generated by Avast Antivirus for SSL/TLS scanning, O=Avast Web/Mail Shield, CN=Avast Web/Mail Shield Root

See this article about Avast.

• https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

cor-el said

Yes, the issuer of the certificate is your security software. Subject C=US, ST=California, L=Mountain View, O=Mozilla Foundation, OU=WebOps, CN=support.mozilla.org Issuer OU=generated by Avast Antivirus for SSL/TLS scanning, O=Avast Web/Mail Shield, CN=Avast Web/Mail Shield Root See this article about Avast.

• https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

Thanks so much! I fixed it. This is what I did with Avast:

Open the dashboard of your Avast application. Go to Settings > Active Protection and click Customize next to Web Shield. Uncheck the Enable HTTPS Scanning setting and confirm this by clicking OK.