Possible bug?
OS X version 10.11.2 FF version 43.0.3
If I change the "Firefox Will" setting in the privacy settings in Preferences in FF from "Remember History" to "Do not remember history" then I get a message prompt to restart FF and after the restart the dock icon shows FF as being open (has a dot underneath it) but FF is not open. If I try and Force Quit the app it does nothing. If I try and open FF via the applications folder I get a message saying FF is not open. If I try and shutdown / restart my Mac it will not shutdown or restart. All other apps work ok. After a hard Shutdown of the Mac I can open FF again but when I close it the problem repeats (i.e. it looks open even though it isn't). This happens on a vanilla install of FF (no extra addons etc.). If I keep the preferences option to "Remember History" then FF opens and closes as normal. The problem is repeatable.
Opaite Mbohovái (5)
For anyone else having this problem the workaround is to not change the privacy settings away from "Remember History". Leaving as is will allow FF to be opened and closed as normal. I guess the "Private Window" option offers the same functionality if required...!
Try to start Firefox in Safe Mode by holding down the Shift (Mac: Options) key to see if that helps.
Added a couple of screenshots to demonstrate what I have said above.
If I try and open FF in safe mode I get the same message saying that FF is not open any more. Strange. I would have expected FF to at least open in safe mode if it is not already open?
Thanks cor-el for your suggestion. I appreciate I am not really asking any questions here but more kind of logging my experiences!!