Is this just with the player? All websites, some, a few?

See if there are updates for your graphics drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Start Firefox in Safe Mode by holding down the <Shift> (Mac=Options) key, and then starting Firefox. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Your help was fine. The following is my followup story only, and is not your fault... I downloaded graphic driver update as suggested (which was already latest), and Dell also updated other drivers. This put me out of business; except for Safe Mode. After 3 days & many hours with tech support, the system is still not right. I'm taking it in stride.