diff -r 000000000000 -r 6474c204b198 toolkit/mozapps/installer/precompile_cache.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/mozapps/installer/precompile_cache.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,94 @@
+/* -*- Mode: Java; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et: */
+/* 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/. */
+
+// see http://mxr.mozilla.org/mozilla-central/source/services/sync/Weave.js#76
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm");
+
+const rph = Services.io.getProtocolHandler("resource").QueryInterface(Ci.nsIResProtocolHandler);
+
+function endsWith(str, end) {
+  return str.slice(-end.length) == end;
+}
+
+function jar_entries(jarReader, pattern) {
+  var entries = [];
+  var enumerator = jarReader.findEntries(pattern);
+  while (enumerator.hasMore()) {
+    entries.push(enumerator.getNext());
+  }
+  return entries;
+}
+
+function dir_entries(baseDir, subpath, ext) {
+  var dir = baseDir.clone();
+  dir.append(subpath);
+  var enumerator = dir.directoryEntries;
+  var entries = [];
+  while (enumerator.hasMoreElements()) {
+    var file = enumerator.getNext().QueryInterface(Ci.nsIFile);
+    if (file.isDirectory()) {
+      entries = entries.concat(dir_entries(dir, file.leafName, ext).map(function(p) subpath + "/" + p));
+    } else if (endsWith(file.leafName, ext)) {
+      entries.push(subpath + "/" + file.leafName);
+    }
+  }
+  return entries;
+}
+
+function get_modules_under(uri) {
+  if (uri instanceof Ci.nsIJARURI) {
+    var jar = uri.QueryInterface(Ci.nsIJARURI);
+    var jarReader = Cc["@mozilla.org/libjar/zip-reader;1"].createInstance(Ci.nsIZipReader);
+    var file = jar.JARFile.QueryInterface(Ci.nsIFileURL);
+    jarReader.open(file.file);
+    var entries = jar_entries(jarReader, "components/*.js")
+                  .concat(jar_entries(jarReader, "modules/*.js"))
+                  .concat(jar_entries(jarReader, "modules/*.jsm"));
+    jarReader.close();
+    return entries;
+  } else if (uri instanceof Ci.nsIFileURL){
+    var file = uri.QueryInterface(Ci.nsIFileURL);
+    return dir_entries(file.file, "components", ".js")
+           .concat(dir_entries(file.file, "modules", ".js"))
+           .concat(dir_entries(file.file, "modules", ".jsm"));
+  } else {
+    throw "Expected a nsIJARURI or nsIFileURL";
+  }
+}
+
+function load_modules_under(spec, uri) {
+  var entries = get_modules_under(uri).sort();
+  // The precompilation of JS here sometimes reports errors, which we don't
+  // really care about. But if the errors are ever reported to xpcshell's
+  // error reporter, it will cause it to return an error code, which will break
+  // automation. Currently they won't be, because the component loader spins up
+  // its JSContext before xpcshell has time to set its context callback (which
+  // overrides the error reporter on all newly-created JSContexts). But as we
+  // move towards a singled-cxed browser, we'll run into this. So let's be
+  // forward-thinking and deal with it now.
+  ignoreReportedErrors(true);
+  for each (let entry in entries) {
+    try {
+      dump(spec + entry + "\n");
+      Cu.import(spec + entry, null);
+    } catch(e) {}
+  }
+  ignoreReportedErrors(false);
+}
+
+function resolveResource(spec) {
+  var uri = Services.io.newURI(spec, null, null);
+  return Services.io.newURI(rph.resolveURI(uri), null, null);
+}
+
+function precompile_startupcache(uri) {
+  load_modules_under(uri, resolveResource(uri));
+}