Sounds to me like some hardware issue, I’ve literally never experienced any of this in the last 5 years on Firefox. My guess is considering it works fine with other browsers the graphics drivers are a bit wonky, or maybe Firefox is falling back to software rendering for some reason. Are you using Linux or Windows?
Strange, usually things just work there considering the limited hardware variety. Is it an older Mac? I’m typing this on an M2 macbook and it works perfectly.
Anyway try to dig into the config and check if you’re using hardware rendering: https://fedoraproject.org/wiki/Firefox_Hardware_acceleration
If it turns out you’re using software rendering try forcing hardware rendering on: https://jamcity.helpshift.com/hc/en/6-genies-and-gems/faq/5737-how-do-i-enable-hardware-acceleration-on-my-browser/
