Who is the issuer of that certificate?

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• Click the link at the bottom of the error page: "I Understand the Risks"

Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".

• Click the "View..." button and inspect the certificate and check who is the issuer of the certificate.

You can see more Details like intermediate certificates that are used in the Details pane.

If "I Understand the Risks" is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and use "This Frame: Open Frame in New Tab".

Note that some firewalls monitor secure (https) connections and send their own certificate instead of the website's certificate.

I have had the same issue with sites like "https://myaccount.yak.ca/" I suspect the certificates that Firefox use have been modified. Have you tried another browser on the same workstation?

It is possible that your system clock has been reset not allowing the certificates to be updated. Open your date and time file and check that the date is correct. This corrected the problem for me. I believe that the 'conduit search' problem was a factor in causing the clock change.