Firefox: child window loses focus because of calling script finishes after opening popup

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Javascript

I have a onDeactivate(evt) js function that get invoked when value of a text box is changed, this function calls another Js function that open a child window(using window.open("....")method and sets focus to it and after opening popup onDeactivate(evt) does some other processings as well.
My question : child window(or popup) does not kept focus in firfox.
In chrome and IE, focus remained with popup.
I infact found that on Mozilla the definition of window.open(https://developer.mozilla.org/en-US/docs/Web/API/Window/open[^]) which states as follows :--
"Note that remote URLs won't load immediately. When window.open() returns, the window always contains about:blank. The actual fetching of the URL is deferred and starts after the current script block finishes executing. The window creation and the loading of the referenced resource are done asynchronously."
In my case remote url belongs to same domain/same application.

On debugging I found that it does set focus on popup window but then when it starts executing parent window scripts it looses its focuse whereas in Chrome, i did observe that url fetch is deferred and only after executing parent window script does it loads and remained focus.

Does anyone know of any setting in firefox that i am missing or any different way of calling a popup in firefox ?