Firefox may display the following error when visiting a website
This error indicates the certificate for the site has been revoked. If you are the site owner you can contact the certificate issuer for further information as to why this might be.
Another possible reason is the distrust of Symantec certificates detailed in our announcement here: Upcoming browser distrust of HTTPS certificates
As the site owner you will need to reissue your certificate to resolve this issue. Please review the following article: How do I reissue my ssl certificate?