See if works from View > Full Screen.

thank you

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

Try to disable hardware acceleration.

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

• https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers

My son came up with a solution; he suggested clicking on a blank portion of the screen, then selecting the full screen icon...it worked!

Don't know why this new 'extra' step is necessary now, but it works. Hope it will help any others that encounter this situation.