Hi,

I’ve had the problem that my firefox screen always opened in the secondary screen in stead of my primary screen.
Online you can find that firefox remembers where you close your screen but this is only partial true.
The latest versions of Firefox only remember which monitor they are on if you close Firefox while it is not maximised.

So the procedure to get firefox to open maximised on the monitor of your choice in dual monitor setup:

1) Un-maximise Firefox window
2) Position un-maximised window on monitor of you choice
3) Close Firefox window
4) Reopen Firefox window
5) Maximise Firefox window
6) Close Firefox window

Now Firefox will open maximised on the screen you chose.

Hopes this helps out you guys.

Cheers,
Sem