That is so weird.

As the first test, 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.

This could be a problem with WebRender or hardware acceleration in Firefox. You can try to disable WebRender to see whether this has effect.

You can try to disable WebRender by setting this pref to true on the the about:config page to see if that has effect.

• gfx.webrender.force-disabled = true

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Thanks to both for the help!!