This article will show you how to troubleshoot the fix SEC_ERROR_UNKNOWN_ISSUER error when you visit some websites using Firefox on Windows.

If you get a "Your connection is not secure" error page and see the error code "SEC_ERROR_UNKNOWN_ISSUER" after you click on Advanced, it means that the certificate provided was issued by a certificate authority that is not known by Firefox and therefore cannot be trusted by default.

This support article will show you how to fix the issue.