How many times have you struggled that user shuld not be able to hit F5 key on your web application?… many times?

IE and chrome are ok to prevent it but firefox has a odd behaviour…

<!-- disable f5 -->
 <script language="javascript" type="text/javascript">
document.onkeydown = function (ev) {

        //firefox
        if (ev)
            if (ev.keyCode == 116) {
                ev.keyCode = 505;
                ev.returnValue = false;
                return false;
            }

        if (window.event && window.event.keyCode == 116) { // Capture and remap F5
            window.event.keyCode = 505;
        }

        if ((window.event && window.event.keyCode == 505) || (window.event.keyCode == 505)) { // New action for F5
            return false;
            // Must return false or the browser will refresh anyway
        }

        return true;
    };
</script>