Call IT, that’s what. That did not help.
Every https page I was trying to access had the same issue and would not load : Error code: SEC_ERROR_UNKNOWN_ISSUER
This has happened to me at least five times and it’s always the same. I had Fiddler running !!!!!!!!!!!!!!! It was intercepting Firefox traffic and my https connections were reporting invalid certs. So, I closed Fiddler and it was fine then.