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

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.

Thank you.

The issue was Avast which I downloaded a couple of days ago. When I deleted it, the problem went away.

Thanks again.

You can keep using Avast as your general security solution, and change one setting: In the "Web Shield" feature, turn off scanning of encrypted connections. Then the Avast Web Shield will only intercept regular HTTP connections (for filtering) and you won't need to set up Firefox to trust Avast getting in the middle of your secure HTTPS connections.

Open the Avast dashboard on the affected system.

Select Settings from the left sidebar menu.
Switch to Active Protection.
Click on Customize next to Web Shield.
Uncheck the "Enable HTTPS Scanning" option and click ok.