Probably a login is required, but if you can share an address to a page a support volunteer can access, we can let you know whether we see the same thing.

Are other HTTPS sites working normally?

Are you sure that this page needs to be accessed via a secure connection and not via an open HTTP connection in case you use an extension to force a secure connection like HTPS Everywhere?

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can click the "Advanced" button to expand this section and show extra details. If the certificate is not trusted because no issuer chain was provided (SEC_ERROR_UNKNOWN_ISSUER) then click the blue error message to expand this section and show the certificate chain. You can click "Copy text to clipboard" and paste this base64 encoded certificate chain text in a reply. That will allow us to details like the issuer of the certificate.

• https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER