firefox SDK builder and MDN

var {Cc, components , Cu} = require("chrome");

Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm");

const WebBrowserPersist = components.Constructor("@mozilla.org/embedding/browser/nsWebBrowserPersist;1",
"nsIWebBrowserPersist");

var persist = WebBrowserPersist();

var targetFile = Services.dirsvc.get("Desk", Ci.nsIFile);
targetFile.append("file.bin");

// Obtain the privacy context of the browser window that the URL
// we are downloading comes from. If, and only if, the URL is not
// related to a window, null should be used instead.

persist.persistFlags = persist.PERSIST_FLAGS_FROM_CACHE
 | persist.PERSIST_FLAGS_REPLACE_EXISTING_FILES;

persist.saveURI('https://forums.mozilla.org/addons/styles/ca_gen2/imageset/logo.png', null, null, null, "", 'c:\\temp\\', null);