This very much sound like a similar issue Linux users are facing from more than a year and it's still not resolved? https://askubuntu.com/questions/1088324/firefox-becoming-unr...
I see a somewhat similar bug on my Linux box, where windows flicker between black and normal, and the flickering stops if i focus on the window. I don't think it's specific to Firefox, though - i believe i've seen all sorts of apps do it. I do have an nvidia graphics card, FWIW.