diff -r 000000000000 -r 6474c204b198 content/base/src/FeedWriterEnabled.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/base/src/FeedWriterEnabled.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,47 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+#include "js/TypeDecls.h"
+#include "nsGlobalWindow.h"
+#include "nsIPrincipal.h"
+#include "nsIURI.h"
+#include "nsString.h"
+#include "xpcpublic.h"
+
+namespace mozilla {
+
+struct FeedWriterEnabled {
+  static bool IsEnabled(JSContext* cx, JSObject* aGlobal)
+  {
+    // Make sure the global is a window
+    nsGlobalWindow* win = xpc::WindowGlobalOrNull(aGlobal);
+    if (!win) {
+      return false;
+    }
+
+    // Make sure that the principal is about:feeds.
+    nsCOMPtr<nsIPrincipal> principal = win->GetPrincipal();
+    NS_ENSURE_TRUE(principal, false);
+    nsCOMPtr<nsIURI> uri;
+    principal->GetURI(getter_AddRefs(uri));
+    if (!uri) {
+      return false;
+    }
+
+    // First check the scheme to avoid getting long specs in the common case.
+    bool isAbout = false;
+    uri->SchemeIs("about", &isAbout);
+    if (!isAbout) {
+      return false;
+    }
+
+    // Now check the spec itself
+    nsAutoCString spec;
+    uri->GetSpec(spec);
+    return spec.Equals("about:feeds");
+  }
+};
+
+}