Is it too late to review the details on the error page? In particular, when you click the Advanced button, there's an error code which may be addressed in this article: How to troubleshoot security error codes on secure websites.

No, it does not even give that kind of error. It literally just says:

Hmm. We’re having trouble finding that site.

We can’t connect to the server at cnrtl.fr.

If that address is correct, here are three other things you can try:

   Try again later.
   Check your network connection.
   If you are connected but behind a firewall, check that Firefox has permission to access the Web.

Last reply by charles.deschenes 2 hours ago

Sorry, current forum security sometimes delays the posting of messages. And in turn, we are not notified of those posts.

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

I ended up uninstalling Firefox, deleting the registry entries, rebooting, then reinstalling. It may be part of the registry corrupted during the upgrade, but I will never know as everything is now working as expected.

Thanks for all your suggestions.

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems.

Problem Solved.