Crossbrowser onbeforeunload?
I found a workaround for Firefox with setTimeout function because it does not have the same behaviour as other web browsers.
window.onbeforeunload = function (e) { var message = "Are you sure ?"; var firefox = /Firefox[\/\s](\d+)/.test(navigator.userAgent); if (firefox) { //Add custom dialog //Firefox does not accept window.showModalDialog(), window.alert(), window.confirm(), and window.prompt() furthermore var dialog = document.createElement("div"); document.body.appendChild(dialog); dialog.id = "dialog"; dialog.style.visibility = "hidden"; dialog.innerHTML = message; var left = document.body.clientWidth / 2 - dialog.clientWidth / 2; dialog.style.left = left + "px"; dialog.style.visibility = "visible"; var shadow = document.createElement("div"); document.body.appendChild(shadow); shadow.id = "shadow"; //tip with setTimeout setTimeout(function () { document.body.removeChild(document.getElementById("dialog")); document.body.removeChild(document.getElementById("shadow")); }, 0); } return message;}GitHub: https://github.com/Aelios/crossbrowser-onbeforeunload
No it does not fire in all browsers. It's not supported in mobile browsers e.g. Safari, Opera Mobile & mini, Dolphin. See Is there an alternative method to use onbeforeunload in mobile safari?
Building upon Tushar Ahirrao solution this works cross browser and triggers once (Works in Firefox, Chrome, whatever)
<html><head><script type="text/javascript">var app = {};app.unloaded = false;app.unload = function() { if (app.unloaded) return; else app.unloaded = true; // your code here return "YO";};</script></head><body onunload="return app.unload();" onbeforeunload="return app.unload();">YO</body></html>Paste above template to empty file then edit it