Interesting, could it be plugins/extensions causing the issue? I've been using Roll20 with Firefox and with Edge for a while without any major issues.
It's usually "ok" and I can deal with performance issues; but these little niggles just makes you want to not deal with it.
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.
How can you possibly use any web browser without an ad blocker installed?