If I search for "wilmott forum" and get taken to https://forum.wilmott.com/ everything isgood
But if I try to go to https://www.wilmott.com/ or http://www.wilmott.com/ I get an error message and cannot access the site
Your connection is not secure
The owner of www.wilmott.com has configured their web site improperly. To protect your information from being stolen, Firefox has not connected to this web site.
This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate.