I have thunderbird open full screen. When opening a new window via Ctrl-N to send an email the below window gets the focus.
This is extremly annoying because thunderbird might perform actions on the list of emails below the new-mail window if
you start typing without changing the focus with a mouse click.
Window behaviou is configured "Activation on mouse contact - priority for mouse".
If I open the new-mail window by mouse click it works as expected – the new window gets the focus.
Sometimes also opening the window by keyboard works but most time it results in focus for the thunderbird main window.

I have ten virtual desktops of which one is used for thunderbird. If I move to that desktop using the keyboard and open
a new mail window using the keyboard the underlying main window receives the focus. If I close the new mail window via
keyboard and directly open a new one via keyboard it receives the focus as expected.
Also if I have sent a mail and open a second new mail window it directly receives the focus.

So the problem only seems to occur if I switch to a different virtual desktop and open a new window via keyboard
shortcut on that desktop.