I updated the link. Thanks for letting us know!

This was a standard error with invalid SSL certificate, which is caused by configuration error on their end. It looks like they moved their website to a different domain name and didn't update their old certificate or set up a redirect from their old domain.

As for if it is "truly dangerous", I think the clearest explanation is that Firefox is warning you of a vulnerability. It doesn't mean anyone is actively trying to take advantage and hack you, so there may not be any immediate danger - however, it means that if someone were acting maliciously, this would give them an opening to do so. It is awesome that Firefox (and other modern browsers) have measures in place to block it when it happens, and is definitely a great idea to keep this browser setting on. If you would like to learn more about the topic, you can look up "HTTP versus HTTPS" for more information.