I understand that there is a security certificate error that mentions that it is from an unknown issuer. To check, right click on the page, and select "Show Page Info"

Next click on the Security tab and show the certificate. Who is the issuer?

If its not someone from this list, this is likely why this is the issue.

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

If you can't inspect the certificate via "I Understand the Risks" then try this:

Open the "Add Security Exception" window by pasting this chrome URL in the Firefox location/address bar and check the certificate:

• chrome://pippki/content/exceptionDialog.xul

In the location field of this window type or paste the URL of the website.

• retrieve the certificate via the "Get certificate" button
• click the "View..." button to inspect the certificate in the Certificate Viewer

You can inspect details like the issuer and the certificate chain in the Details tab of the Certificate Viewer. Check who is the issuer of the certificate. If necessary then you can attach a screenshot that shows the certificate viewer.