There is security software like Avast and Kaspersky and BitDefender and ESET that intercept secure connections and send their own certificate.

https://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

hi robin, your website might not provide a full certificate chain from an intermediate certificate to a trusted root certificate. you could check your site with a tool like https://www.ssllabs.com/ssltest/ and refer to the documentation of your certificate provider on how to properly implement it.

We have seen reports that the time as set via an internet based time service was so much out of sync that an error occurs, so if that is the case with your current service then try to switch to another time service.

Try to rename the cert8.db file (cert8.db.old) and delete the cert_override.txt file in the Firefox profile folder to remove intermediate certificates and exceptions that Firefox has stored.

If that has helped to solve the problem then you can remove the renamed cert8.db.old file.

Firefox will automatically store intermediate certificates that servers send in the Certificate Manager for future use.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox