// предварительные определения
function getContent (aReq, aCont)
{
    var st;
    aReq. cancel (st);
    if (this. redFlag)
    {
        // обрабатываем редирект
    }
    else
    {
        var x = new XMLHttpRequest;
        ... 
    }
}

function chanListener (a, b, c) { this. redFlag = false; }
chanListener. prototype = bookmarksFavIconLoadListener. prototype;
chanListener. prototype. onRedirect = function (aCh, aNCh) { this. redFlag = true; }
chanListener.prototype. onDataAvailable = function (aReq, aCont, aIS, aOff, aC) {}
chanListener. prototype. onStartRequest = getContent;

// собственно обработка
var kIOContractID = "@mozilla.org/network/io-service;1";
var kIOIID = Components. interfaces. nsIIOService;
var IOSVC = Components. classes[kIOContractID]. getService(kIOIID);
var chan = IOSVC. newChannel ("http://gmail.google.com/gmail", null, null);
var listener = new chanListener ("http://gmail.google.com/gmail", "", chan);
chan. notificationCallbacks = listener;
chan. asyncOpen (listener, null);

with (Components)
with (interfaces)
function myXMLHttpRequest ()
{
    var mxhr = this;
    var request, reqId;
    var observer =
    {
        observe: function (subject, topic, data)
        {
            subject. QueryInterface (nsIHttpChannel);
            if ((reqId == subject. notificationCallbacks. requestId) && mxhr. onresponse)
            {
                try
                {
                    mxhr. onresponse (subject);
                } catch (e) {}
            }
        }
    }
    function removeObserver ()
    {
        var rdfService = classes ["@mozilla.org/rdf/rdf-service;1"].
                         getService (nsIRDFService);
        rdfService. UnregisterResource (reqId);
        try
        {
            var observerService = classes ["@mozilla.org/observer-service;1"].
                                  getService (nsIObserverService);
            observerService. removeObserver (observer, "http-on-examine-response");
        } catch (e) {}
    }
    function onError () 
    {
        removeObserver ();
        if (mxhr. onerror)
            mxhr. onerror ();
    }
    function onLoad ()
    {
        removeObserver ();
        if (mxhr. onload)
            mxhr. onload ();
    }
    this. open = function (method, url, asyncFlag)
    {
        var rdfService = classes ["@mozilla.org/rdf/rdf-service;1"].
                         getService (nsIRDFService);
        reqId = rdfService. GetAnonymousResource (). Value;
        request = new XMLHttpRequest ();
        request. open (method, url, asyncFlag);
        request. requestId = reqId;
        request. onerror = onError;
        request. onload = onLoad;
    }
    this. send = function (message)
    {
        if (mxhr. onresponse)
        {
            var observerService = classes ["@mozilla.org/observer-service;1"].
                                  getService (nsIObserverService);
            observerService. addObserver (observer, "http-on-examine-response", false);
        }
        request. send (message);
    }
    this. setRequestHeader = function (name, value)
    {
        request. setRequestHeader (name, value);
    }
}


// использование
var req = new myXMLHttpRequest ();
req. open ("GET", "http://www.google.com", true);
req. onresponse = function (httpChannel)
{
    print (httpChannel. responseStatus);
    // if (httpChannel. responseStatus == 302) // redirect
    // {
    //     . . .
    // }
}
req. send ();