Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

check this:

• see if there are updates for your graphics drive drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

You can try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

You need to close and restart Firefox after toggling this setting.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

You can try a different Windows theme and Windows background image (wallpaper) to see if that helps.

FredMcD & cor-el --

   Thanks for the suggestions. Gave most of them a try. Don't have Flash installed so that was not an issue. Also, ASUS has not updated its version of the Intel vid driver sin W10 came out, so I wondered of THAT might be an issue. Ended up diving in a little deeper and noticed that the newer W10 drivers that got pushed changed the way that some of my about:config settings responded compared to W7/8.1.  I reverted some of the gfx.direct2d... settings to default, and the problem cleaned itself up.  Appreciated your quick feedback!

KalvanOfOtherwhen

That was very good work. Well Done.