Start Firefox in [[Safe Mode]] to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). * Don't make any changes on the Safe mode start window. See: * [[Troubleshooting extensions and themes]] 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")