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.

Solution 1:

It seems that the key setting to resolve this is "When switching to an application, switch to a Space with open windows for the application."

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.