How to get browser close Event in firefox using javascript ?

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Javascript

Hi all,

I have to check some condition on browser close event means When i close the browser ( when i click on [x] ) that event i want using javascript.
i am able to do this in IE but in firfox itis not working properly
i tried

var altKey  = false;
var keyCode = 0;

function closeSession(evt){

    evt = (evt) ? evt : event;

    clickY  = evt.clientY;
    altKey  = evt.altKey;
    keyCode = evt.keyCode;

    if(!evt.clientY){
        // Window Closing in FireFox
        // capturing ALT + F4
        keyVals = document.getElementById('ffKeyTrap');
        if(keyVals.value == 'true115'){
            return 'close 1';
        }

        if(keyVals.value == ''){
            // capturing a window close by "X" ?
            // we have no keycodes
            return 'close 2';
        }

    } else {
        // Window Closing in IE
        // capturing ALT + F4
        if (altKey == true && keyCode == 115){
            alert('close 1');
        // capturing a window close by "X"
        } else if(clickY < 0){
            alert('close 2');
        // simply leaving the page via a link
        } else {
            //alert('close 3');
            return void(0);
        }
    }
}

function whatKey(evt){
    evt = (evt) ? evt : event;
    keyVals = document.getElementById('ffKeyTrap');
    altKey  = evt.altKey;
    keyCode = evt.keyCode;
    if(altKey && keyCode == 115){
        keyVals.value = String(altKey) + String(keyCode);
    }
}

window.onkeydown      = whatKey;
window.onbeforeunload = closeSession;