This issue can be caused by an extension that isn't working properly. Start Firefox in <u>Safe Mode</u> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem. *switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance *do NOT click the "Refresh Firefox" button on the Safe Mode start window *https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems