Howto keep child window always on the top of parent window

Hi,
I have a basic question as follows.
For example, In Firefox, click the menu File/OpenFile to open a child window.
I want to keep this kind of child window always on the top of the parent window, since this child window exclusively gets the input of the whole program.
How to achieve this?
Thanks.

We call these modal windows and you want:

Style * RaiseTransient

– Thomas Adam

It works.
Thank you, Adam.