在Mozilla Firefox Add-on中的overlay.js中重新定义XMLHTTPRequest

appcontent.addEventListener("load", function(event){
        var original_xhr = XMLHttpRequest;
        XMLHttpRequest = function () {
            original_xhr();
            alert ("I redefined this function");
        }
        alert ("Page loading");
    } , true)

try
{
    var callbacks = httpChannel.notificationCallbacks;
    if (callbacks) {
        var xhrInterface = callbacks.getInterface(Components.interfaces.nsIXMLHttpRequest);
        // XHR REQUEST
    }
}
catch (exc)
{
    if (exc.name == "NS_NOINTERFACE") {
        // NO XHR REQUEST
    }
}