Firefox keeps opening on wrong screen I use a 4K monitor as my main monitor, with my laptop off to one side.  Nearly every app I open opens correctly on the 4K monitor.
But Firefox always opens on the laptop monitor, even when its windows were on my 4K monitor when closed.  Also, when I make a new window in Firefox, it opens on the laptop monitor (even when all existing Firefox windows are in the 4K monitor).  Needless to say, this is very annoying.
How can I configure Firefox (or my Mac OS) to correctly open new windows on the 4K monitor like all other apps do?
This is on Mac OS 10.11.6 and Firefox 78.1.0esr.
It seems that the key setting to resolve this is &quot;When switching to an application, switch to a Space with open windows for the application.&quot;
With that box CHECKED, new windows open where I expect them to.
Closing Firefox with two windows open both on my 4K display and then reopening Firefox, now reopens one window on the built-in display and one window on the 4K display, which is weird.  I may post further updates if I figure out why or figure out how to resolve that.  But having NEW windows open on my 4K display (as they now do) reduces the annoyance factor by a LOT.