Change confirmation message for Firefox

Hi!

I use Window.ClosingEvent. When I try change confirmation message

    

    public void onApplicationClosing(ApplicationClosingEvent event) {

                event.setMessage("bla-bla");

    }

It's not working for Firefox. Firefox show me default message: "This page is asking you to confirm that you want to leave - data you have entered may not be saved."

How can I change this message?

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