Armin's answer is so useful, thank you. #2 is what's most important to know when trying to set up unload events that work in most browsers: you cannot alert() or confirm(), but returning a string will generate a confirm modal.
But I found that even with just returning a string, I had some cross-browser issues specific to Mootools (using version 1.4.5 in this instance). This Mootools-specific implementation worked great in Firefox, but did not result in a confirm popup in Chrome or Safari:
window.addEvent("beforeunload", function() {
return "Are you sure you want to leave this page?";
});
So in order to get my onbeforeonload event to work across browsers, I had to use the JavaScript native call:
window.onbeforeunload = function() {
return "Are you sure you want to leave this page?";
}
Not sure why this is the case, or if it's been fixed in later versions of Mootools.