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

Slowness caused by Norton 360 Safe Web. I used Norton to scan my installation and they said it was okay, so I do not know why it happened. I may have turned on hardware acceleration yesterday when trying to fix it, so I am going to try to turn it off if I can find it again.

How to disable Hardware Acceleration {web link}