Object.defineProperty(unsafeWindow, "onbeforeunload", {
    get: function() {
        return null;
    },
    set: function() {
        console.log("user script \"Disable onbeforeunload\" blocked window.onbeforeunload = ...");
    }
});

// disable window.addEventListener("beforeunload", ...);
var oldAEL = unsafeWindow.addEventListener;
unsafeWindow.addEventListener = function() {
    if (arguments[0] === "beforeunload") {
        console.log("user script \"Disable onbeforeunload\" blocked window.addEventListener(\"beforeunload\", ...)");
    } else {
        oldAEL.apply(unsafeWindow, arguments);
    }
}