That issue can be caused by an extension that isn't working properly. *https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes Start Firefox in <u>[[Safe Mode]]</u> 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.com/kb/Safe+Mode If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears. *Use "Disable all add-ons" on the [[Safe mode]] start window to disable all extensions. * Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")