I am running Firefox on Ubuntu 20.4

Could you try disabling Firefox from using hardware acceleration? This is a potential workaround for incompatibilities between Firefox and your graphics card/chipset driver software. This article describes where to make that change: [[Firefox's performance settings]]. This will take effect the next time you exit Firefox and start it up again. Any difference? Since hardware acceleration improves the appearance of fonts and animations, you may want to check whether graphics card/chipset driver updates are available for your system. This article suggests safe ways to do that: [[Upgrade your graphics drivers to use hardware acceleration and WebGL]].