What is your computer system and Firefox? Start '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Firefox in Safe Mode]''' {web Link} by holding down the '''<Shift><br> ''(Mac=Options)'' ''' key, and then starting Firefox. A small dialog should appear. Click '''Start In Safe Mode''' (not Refresh). Is the problem still there?

Try to uninstall and reinstall Firefox. You can find the full version of the current Firefox release (47.0) in all languages and all operating systems here: *https://www.mozilla.org/en-US/firefox/all/ See also: *http://kb.mozillazine.org//Installing_Firefox#Mac_OS_X ---- Create a new profile to test if your current profile is causing the problem. See "Creating a profile": *https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems. *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox