Try to disable OMTC and leave hardware acceleration in Firefox enabled.

• about:config page: layers.offmainthreadcomposition.enabled = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

You can check if there is an update for your graphics display driver or otherwise disable hardware acceleration.

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

I am not computer literate so therefore your instructions are not a help, for me. Thank you, anyway, for your help.

You can check if you can start Firefox in Safe Mode by holding down the Shift key.

• https://support.mozilla.org/kb/Safe+Mode

You can try to disable hardware acceleration in Firefox if that helped.

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