Mozilla Firefox won't force quit - won't show any windows or load a page

If you use Firefox, this is a bug that can crop up on all platforms (MacOs, Linux and Windows)

The symptom is Firefox appears to be running, however it won't display any windows and is unresponsive. It can't even be force-quit.

I've seen this happen in Windows, and in MacOS. I haven't yet seen it happen in Linux, but I hear it can.

Each tab or window you have open in Firefox creates a new process - it's like running a new copy of Firefox for each open website. Sometimes, this process can get stuck and 'run away' with your processor's CPU usage.

To fix it, do a full system restart (under MacOS / OSX, make sure the same programs don't auto-relaunch on startup if asked). You may even find your computer refuses to restart when this happens. You might have to force turn it off and back on again. Once your computer has started back up, open Firefox. It should open this time. Before you go to any other website, type this in the browser address bar.

Press return to load the page (This loads a hidden Firefox configuration page). Accept the risk and continue:

You should now have a page that looks like this:

Click 'Show All' at the top of the page.

You should now have a page that looks like this:

You are looking for the value named:

Change the value from 'True' to 'False'

Next, look for the value named:

Again, change the value from 'True' to 'False'

Now Quit Firefox, reopen and it should not hang on you like this anymore.