Was anything in this article helpful (I hope the question form suggested it based on your subject line): [[How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites]] If not, can you share one of the certificates that Firefox objects to? If you click the SEC_ERROR_UNKNOWN_ISSUER code, a panel should open. Hopefully it displays a coded version of the certificate (large block of gibberish) and you can use the copy button to capture it and then paste it into a reply. ''If the certificate is missing:'' Try viewing the certificate on a site with lower security demands, i.e., one where you can see the "Add Exception" button. For example: https://www.userchrome.org/ Click the Advanced button again, then click the "Add Exception" button (see first screenshot below). We're not really going to add an exception, but we can get a better view of the certificate here. In the Add Exception dialog, click the "View" button to pop up a certificate viewer. The interesting part is the "Issued by" section. I've marked that on the second screenshot for reference. On my site, the "Issued by" section shows me "Let's Encrypt Authority X3". What do you see there? (These example screenshots are from a test page, not my site) <img src="https://prod-cdn.sumo.mozilla.net/uploads/images/2018-01-26-20-20-02-900b0c.png" width="500"> <img src="https://prod-cdn.sumo.mozilla.net/uploads/images/2018-01-26-20-20-27-291465.png" width="500">