Firefox keeps opening on wrong screen
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.