I tried reading up a bit on reasons why such a warning happens and the only concrete thing I could find that would trigger it is an issue with certifications, whether they’re expired or self signed. I found an article about FF making just such errors more specific so there’s now one that warns for man in the middle attacks specifically and others for other particular issues. They all used to be a generic “not safe” warning up until some point last year.
So the fact that you’re getting a generic not safe warning means it’s not one of the greater specific threats as far as I can tell. But since the certificate is active and verified I really don’t know. I’m running FF Quantum up to date version 68.0 (32bit).
The site also doesn’t demand any real information out of you to use it. Don’t have to log in to your steam account, just put your name/ID into it. So I don’t see what if anything it could possibly try to steal.