download.jsm + Addon SDK - https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Downloads.jsm

io.js

/*
 Copyright 2012 Christopher Hoobin. All rights reserved.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
 met:

 1. Redistributions of source code must retain the above copyright
 notice, this list of conditions and the following disclaimer.

 2. Redistributions in binary form must reproduce the above
 copyright notice, this list of conditions and the following
 disclaimer in the documentation and/or other materials provided
 with the distribution.

 THIS SOFTWARE IS PROVIDED BY CHRISTOPHER HOOBIN ''AS IS'' AND ANY
 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL CHRISTOPHER HOOBIN OR
 CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 The views and conclusions contained in the software and documentation
 are those of the authors and should not be interpreted as representing
 official policies, either expressed or implied, of Christopher Hoobin.
 */
var { Ci,Cu,Cc } = require('chrome');
var data = require("sdk/self").data;
const { defer } = require('sdk/core/promise');
Cu.import("resource://gre/modules/Services.jsm");
Cu.import("resource://gre/modules/Downloads.jsm");
Cu.import("resource://gre/modules/osfile.jsm")
Cu.import("resource://gre/modules/Task.jsm");

/**
 * ``saveURL`` Util
 * @param {Window} window
 * @param {nsIURL} sourceURI - download url
 * @param {nsIURL} fileURI - path for save
 * @param {string} filename - filename for save
 * @return {promise} promise
 */
var saveFile = function (window, sourceURI, fileURI, filename) {
    var deferred = defer();
    Task.spawn(function () {
        let list = yield Downloads.getList(Downloads.ALL);

        let view = {
            onDownloadChanged: function (aDownload) {
                aDownload.whenSucceeded().then(function () {
                    var limitSize = 1000 * 1000;// 1000 * 1000Byte
                    if (aDownload.totalBytes > limitSize) {
                        deferred.resolve(aDownload);
                    } else {
                        deferred.reject(new Error(aDownload));
                    }
                });
            }
        };
        yield list.addView(view);
        try {
            let download = yield Downloads.createDownload({
                source: sourceURI,
                target: fileURI
            });
            list.add(download);
            try {
                download.start();

                var notifications = require("sdk/notifications");
                notifications.notify({
                    title: "Download Download",
                    text: "Start " + filename,
                    iconURL: data.url("icon.ico")
                });
            } catch(e) {
                deferred.reject(e);
            }
        } catch (e){
            deferred.reject(e);
        }

    }).then(null, Cu.reportError);

    return deferred.promise;
};


exports.saveFile = saveFile;