Firefox can't even keep up/not tested by developers. We sometimes use roll20 to game, but things like blank screens make it unusable. I've had to just use chromium and not worry about it.
Our last game was about 2 weeks ago... two of us hit the same issue with the screen not "repainting" and being black. No issue with the chrome browsers, so it's easier to just switch rather than bog the game down with debugging/diagnosis.
It's usually "ok" and I can deal with performance issues; but these little niggles just makes you want to not deal with it.
Did you have an ad blocker or other privacy add-ons installed? Many "website doesn't work in Firefox" issues are related to ad blockers.
OTOH, if the page is black and not repainting, that might be a graphics issue (in Firefox or the GPU driver). Firefox uses the GPU more than Chrome for rendering and video decoding, which can be great for performance but unfortunately means users can be exposed to GPU driver bugs.
