This could be a problem with WebRender, so you can try to disable WebRender to see whether this has effect. You can disable WebRender by setting this pref to true on the the <b>about:config</b> page. *gfx.webrender.force-disabled = true You can open the <b>about:config</b> 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 ----- You can try to disable hardware acceleration in Firefox. *Settings -> General: Performance<br>remove checkmark: [ ] "Use recommended performance settings"<br>remove checkmark: [ ] "Use hardware acceleration when available" *https://support.mozilla.org/en-US/kb/performance-settings Close and restart Firefox after modifying the setting to make changes effective. *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems You can check if there is an update for your graphics display driver and check for hardware acceleration related issues. *https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

To disable hardware acceleration was the solution. Now it works fine. Thanks a lot for your help, Ludwig