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

I think it was because I had the Firefox for a long time on and used up much of my RAM, I have 8GB though is there any way to prevent Firefox from using so much RAM?

Ok Next time you submit a Question please turn on send info to Mozilla as it gives us stuff to work with. By not doing this we are shooting blind at stuff. Thank You.

See if you have Safe Browsing Version 4 extension is part of a Mozilla experiment, called 'Shield Studies'. Without even knowing it, you may have volunteered for this Mozilla experiment. Would you go to : Options => Privacy & Security => under 'Data Collection and Use' : Did you uncheck : 'Allow Firefox to install and run studies ' or is it checked  ?

To check if you are enrolled in these studies : Type in the address bar about:studies then enter

Any extensions that you see listed there, you can either disable or remove, just like this Safe Browsing version 4 (at the end of the experiment they will be disabled anyway).

Check from the address car (copy/paste) about:support then enter and look to see if Multi Processor is turned on. Multi-processor support feature may be disabled depending on your setup (it was for me) - you can check this by launching Firefox, then enter in "about:support" in the address bar; near the bottom, you should see a heading that says: "Multiprocess Windows", then look at the value next to it. If it says "0/1", then that means it's disabled.

If is 0/2 or higher

You can try to modify multi-process settings to see if this has effect.

   set dom.ipc.processCount to 1 if it is currently set to a higher value (4)
   disable multi-process windows in Firefox 

You can disable multi-process windows in Firefox by setting these prefs to false on the about:config page.

   browser.tabs.remote.autostart = false
   browser.tabs.remote.autostart.2 = false 

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

http://kb.mozillazine.org/about:config

http://www.ghacks.net/2016/07/22/multi-process-firefox/

Try : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Use Refresh...............

Note : Firefox 56.0.2 is current release https://www.mozilla.org/firefox/all/

Note : Firefox 57 Quantum release date Nov 14th https://www.mozilla.org/en-US/firefox/quantum/ Note : Legacy Extensions will be disabled and or removed.

Please let us know if this solved your issue or if need further assistance.