window.onbeforeunload and window.onunload is not working in Firefox, Safari, Opera?

Unfortunately, the methods you are using are unsupported in those browsers. To support my answer (this unsupportive behaviour) I have given links below.

onbeforeunload and onunload not working in opera... to support this

onbeforeunload in Opera
http://www.zachleat.com/web/dont-let-the-door-hit-you-onunload-and-onbeforeunload/

Though the onunload event doesn't work completely, you can use onunload to show a warning if a user clicks a link to navigate away from a page with an unsaved form.

onunload not working in safari... to support this

https://www.webkit.org/blog/516/webkit-page-cache-ii-the-unload-event/

You could rather try using the pagehide event in the safari browser in lieu of onunload.

onunload not working in firefox... to support this

https://bugzilla.mozilla.org/show_bug.cgi?id=681636

They are yet to come up with a solution in FF too

Wish you good luck cheers.

Here is the working solution for ie, firefox and chrome:

var myEvent = window.attachEvent || window.addEventListener;
var chkevent = window.attachEvent ? 'onbeforeunload' : 'beforeunload'; /// make IE7, IE8 compitable

            myEvent(chkevent, function(e) { // For >=IE7, Chrome, Firefox
                var confirmationMessage = 'Are you sure to leave the page?';  // a space
                (e || window.event).returnValue = confirmationMessage;
                return confirmationMessage;
            });

I was able to get it to work in IE and FF with jQuery's:

$(window).bind('beforeunload', function(){

});

instead of: unload, onunload, or onbeforeunload