Sub window on secondary screen

When I open a new application such as Firefox, it opens on the primary screen as expected. But when a sub window opens, for example when I go to save a picture from Firefox, the save window opens on the secondary screen. Now if I have dragged Firefox to the secondary screen having the sub window there would make sense, but not when the main window is on the primary screen. This is also happening from Thunderbird. Any suggestions on how to keep the sub window with its parent would be much appreciated, thank you in advance!
p.s. yes I have my big monitor set as the primary screen, and the built in monitor set as secondary.

Try to have that Application open on your second screen, before you click or nudge some File to open with it.

Programs remember the Position where there was before closing… and reopen that file or Application on that position. I searched but i am not sure if there is a command line option (to make some short cut script) to open it every time at that position. I am just sure it have to be a solution out there for your issue.