How get main browser window HWND under Windows to move and lock focus on NPAPI based plug-in window?

I wrote plug-in that called from JavaScript method.

Plug-in require user interaction and must be separate
window (do not built-in into page).

After filling data to window it closed and to JS returned
string as result of plug-in job.

I wont make plug-in window stay on top of browser window
and restrict from focusing to browser window until
user close plug-in window by passing browser HWND
to CreateWindowExW(__in_opt HWND hWndParent) call.

Such behavior show alert window when executed 'alert()'
call from JS: until you click 'OK' main browser window inactive.

But I don't know how get browser HWND.

I develop plug-in for Firefox, Opera, Chrome and Safary,
so any portable hints can be very useful.

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Best regards!