Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences). "Clear the cache": *Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now" "Remove the cookies" from websites that cause problems. *Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies" Start Firefox in <u>[[Safe Mode|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/kb/Safe+Mode *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes