What were you doing at the time? surfing, watching a clip, reading something . . . . [https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode Start Firefox using Troubleshoot(Safe) Mode] {web link} A small dialog should appear. Click '''Start In Troubleshoot(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