Firefox/Mozilla doesn't certify certificate that is done by another organization and if the certificate is invalid or out of date then it's up to one owning the certificate to update to allow access with the certificate to be used. Ad Norton also gave you the same error message so this isn't a Firefox issue.

Mark, Thank you for the reply. Unfortunately your reply does not help me solve my issue. I did not include anything that says Norton gave me the same error message. As far as I can tell a Norton Certificate (which date appears valid) is in the W10-Pro Trusted certificate list as well as within the Firefox certificate list. Currently I am not trusting (using) Firefox with any accounts that require passwords.

If not Norton - Firefox isn't the issuing certificate it's verifies it's up to date and matches certification that should be for site or whom issuing it and if it fails then the Browser will protects itself from malicious certificates. So you should ask the site that uses to do their proper checks.

My apologizes, but I still don't understand your response.

I open a URL with FIrefox in protected mode, the page opens and I click on the lock icon, I check that the connection is secure and who it is verified by. If I'm happy with the verifier I continue.

Over the last couple of weeks its almost always Norton is the verifier. Click on the lock, Firefox responds with three lines of text. It tells me "You are securely connected to this site". next line Verified by: Norton Web/Mail Shield. In the next line Firefox says: Mozilla does not recognize this certificate issuer. It may have been added from your OS or by an administrator.

The certificate managers for Microsoft W10-Pro and Firefox both show Norton Web/Mail Shield Root with a date 1/1/2010 - 1/1/2040.

So my dilemma is: why does Firefox permit me access to the URL when it has no recognition of the certificate issuer?

This morning, using Firefox I opened several URL's and everyone was certified by Norton. I moved to Microsoft Edge and opened the same URL's and got a variety of certifiers, none were Norton.

Some sites permits access but it does say use at your own risk. Without a url of the problem site no one will know why it's doing that. How Norton verifies that's Norton not Firefox.