OP: Hmmm, wonder what the problem is if other people or some computers aren't having the same issues with same O.

But just now, I decided to "roll back" Firefox. I downloaded a folder for 50.0.2 from Mozilla, and the .exe file opened that version's Firefox window without installing anything. When I go to FKG, everything is back to normal, no more graphical issues. So maybe I'll just use the old stand-alone version only for Nutaku games.

Except I don't have enough stamina yet to see if the loading issues persist. If the loading problem continues, I'll update here.