Took me ages to figure this out but it's actually very simple.
"Un-maximize" Firefox so it's not taking up the whole screen, move it to the place you want it to open, then close it. Do NOT maximize it first. Make sure all four edges are neatly inside the monitor you want it to live in, then close it.
When you open it it will be where you left it. Now you can maximize it and close it. It will remember it's location and open, maximized, in that monitor next time.
Hope this helps you.