diff -r 000000000000 -r 6474c204b198 mobile/android/modules/WebappManagerWorker.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mobile/android/modules/WebappManagerWorker.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+importScripts("resource://gre/modules/osfile.jsm");
+
+function log(message) {
+  dump("WebManagerWorker " + message + "\n");
+}
+
+onmessage = function(event) {
+  let { url, path } = event.data;
+
+  let file = OS.File.open(path, { truncate: true });
+  let request = new XMLHttpRequest({ mozSystem: true });
+
+  request.open("GET", url, true);
+  request.responseType = "moz-chunked-arraybuffer";
+
+  request.onprogress = function(event) {
+    log("onprogress: received " + request.response.byteLength + " bytes");
+    let bytesWritten = file.write(new Uint8Array(request.response));
+    log("onprogress: wrote " + bytesWritten + " bytes");
+  };
+
+  request.onreadystatechange = function(event) {
+    log("onreadystatechange: " + request.readyState);
+
+    if (request.readyState !== 4) {
+      return;
+    }
+
+    file.close();
+
+    if (request.status === 200) {
+      postMessage({ type: "success" });
+    } else {
+      try {
+        OS.File.remove(path);
+      } catch(ex) {
+        log("error removing " + path + ": " + ex);
+      }
+      let statusMessage = request.status + " - " + request.statusText;
+      postMessage({ type: "failure", message: statusMessage });
+    }
+  };
+
+  request.send(null);
+}