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Obsolete since Gecko 60 (Firefox 60 / Thunderbird 60 / SeaMonkey 2.57)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

nsIXMLHttpRequest along with nsIJSXMLHttpRequest and nsIXMLHttpRequestEventTarget are Mozilla's implementation details of the DOM XMLHttpRequest object.

Note: If you're a web developer or a Mozilla add-on developer, please refer to the XMLHttpRequest documentation instead.

This page contains documentation, specific to Mozilla application and add-on developers.

The interface definition: https://dxr.mozilla.org/mozilla-central/source/dom/xhr/nsIXMLHttpRequest.idl

Elevated Privileges

As mentioned in the "Non-Standard Properties" the property of channel was read-only. When using the XPCOM interface, as seen below in Example 2, we can get access to this. The most obvious benefit is that we can set nsiRequest - Constants in the xhr.channel.loadFlags. For instance, as done in Example 2, the flag of LOAD_ANONYMOUS is added, this strips all user data (cookies, tokens, etc).

Using event handlers from native code

(Not sure if it's up-to-date)

From native code, the way to set up onload and onerror handlers is a bit different. Here is a comment from Johnny Stenback <jst@netscape.com>:

The mozilla implementation of nsIXMLHttpRequest implements the interface nsIDOMEventTarget and that's how you're supported to add event listeners. Try something like this: nsCOMPtr<nsIDOMEventTarget> target(do_QueryInterface(myxmlhttpreq)); target->AddEventListener(NS_LITERAL_STRING("load"), mylistener, PR_FALSE) where mylistener is your event listener object that implements the interface nsIDOMEventListener. The 'onload', 'onerror', and 'onreadystatechange' attributes moved to nsIJSXMLHttpRequest, but if you're coding in C++ you should avoid using those.

Though actually, if you use addEventListener from C++ weird things will happen too, since the result will depend on what JS happens to be on the stack when you do it....

Conclusion: Do not use event listeners on XMLHttpRequest from C++, unless you're aware of all the security implications. And then think twice about it.

Example code

This is a simple example code for opening a simple HTTP request from a xul application (like a Mozilla extension) without using observers:

 var req = Components.classes["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance();
 req.open('POST', "http://www.foo.bar:8080/nietzsche.do", true);

Example 2

var {Cu: utils, Cc: classes, Ci: instances} = Components;
function xhr(url, cb) {
    let xhr = Cc["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance(Ci.nsIXMLHttpRequest);

    let handler = ev => {
        evf(m => xhr.removeEventListener(m, handler, !1));
        switch (ev.type) {
            case 'load':
                if (xhr.status == 200) {
                Services.prompt.alert(null, 'XHR Error', 'Error Fetching Package: ' + xhr.statusText + ' [' + ev.type + ':' + xhr.status + ']');

    let evf = f => ['load', 'error', 'abort'].forEach(f);
    evf(m => xhr.addEventListener(m, handler, false));

    xhr.mozBackgroundRequest = true;
    xhr.open('GET', url, true);
    xhr.channel.loadFlags |= Ci.nsIRequest.LOAD_ANONYMOUS | Ci.nsIRequest.LOAD_BYPASS_CACHE | Ci.nsIRequest.INHIBIT_PERSISTENT_CACHING;
    xhr.responseType = "arraybuffer"; //dont set it, so it returns string, you dont want arraybuffer. you only want this if your url is to a zip file or some file you want to download and make a nsIArrayBufferInputStream out of it or something

xhr('https://www.gravatar.com/avatar/eb9895ade1bd6627e054429d1e18b576?s=24&d=identicon&r=PG&f=1', data => {
    Services.prompt.alert(null, 'XHR Success', data);
    var file = OS.Path.join(OS.Constants.Path.desktopDir, "test.png");
    var promised = OS.File.writeAtomic(file, new UInt8Array(data));
        function() {
            alert('succesfully saved image to desktop')
        function(ex) {
             alert('FAILED in saving image to desktop')