Yes, there's a problem with the security certificate because the forum isn't hosted on a private server. It would work there, but that would make things much slower, more work, etc.
If I'd had to do it again, I'd leave the forum under http only, and forget about https, but it is supposed to be safer wrt privacy and malware etc.
Either way, there is no problem with the forum's security (AFAIK), so either let Firefox make an exception, or access the forum through http.