I had the SEC_ERROR_UNKNOWN_ISSUER problem for a few months. I just reinstalled completely Windows and the problem was gone. Now 2 days later I get this message again when I want to go on sites like google, amazon, youtube, etc.
I don't use any antivirus but I runned a scan with Windows Defender and Clam and found nothing.
I just upgraded to the new Firefox relase and the problem remains.
Read all the articles about this issue and found no solution. The problem doesn't appear with other web browsers such as Microsoft Edge.
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You can check details about the issuer of the certificate and the certificate chain.
- click the "Advanced" button on the error page to show more detail
- click the blue "View Certificate" text to inspect the certificate chain in the Certificate Viewer
You can check the connection settings.
- Options/Preferences -> General -> Network: Connection -> Settings
If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.
See "Firefox connection settings":