Hey there Crow,
In general Firefox isn't the best browser to run games in. Since the vast majority of web browsers are based on the blink engine most games are developed and optimized specifically for it. Firefox doesn't use the blink engine, it uses gecko. I suspect that's why you're having problems. I've also had problems with Firefox being laggy or glitchy when running the various things.
You might want to try a chromium based browser such as edge, brave, or chrome. Personally I've had great results when running a couple web games in Vivaldi, I highly recommend.