Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there? https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding Could try this as a new install : This the best way to uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles Reinstall with Current Release Firefox 57.0 with a Full Version Installer https://www.mozilla.org/firefox/all/ Going back : unfortunately 56.0.2 is no longer safe to use for every day use, but 52.5.0 ESR is! It will continue to get security updates until May 2018, and you can download and install it from this page: https://www.mozilla.org/en-US/firefox/organizations/all/ Note : Legacy Extensions will be deleted or removed in any version update after May 2018. You should make a backup of your Profile before going back and just because: https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles Please let us know if this solved your issue or if need further assistance.

I had to reinstall Firefox. Safe mode didn't help. At least it is working now. Thank you.