This is the screen message.

Secure Connection Failed

An error occurred during a connection to www.facebook.com. Peer's certificate has an invalid signature. (Error code: sec_error_bad_signature)

   The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
   Please contact the website owners to inform them of this problem.

You can try to set security.use_mozillapkix_verification to false on the about:config page as a test to see if that has effect.

https://support.mozilla.org/en-US/questions/1012765#answer-609064

There is security software like Avast and Kaspersky and BitDefender and ESET 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.