Subject line says it all. Firefox has been reporting for at least a week that the forum subset has an expired security certificate and it blocks access to these forums. Safari, however, does not. I prefer to use Firefox because of all the ad blockers, anti-trackers, and spam detectors add-ons that run there to protect me from scams and adverts. I only use Safari as now to report when the strictness of Firefox becomes an inconvenience.
Looks like the issue has been resolved, thanks @Dan_Dascalescu and @Agaricus!
Thanks to @Dan_Dascalescu and @Agaricus for fixing the problem.