new file mode 100644
--- /dev/null
+++ b/im/app.mozbuild
@@ -0,0 +1,13 @@
+# 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/.
+
+app_libxul_dirs = []
+app_libxul_static_dirs = []
+
+# Invoked from /mozilla.
+if not CONFIG['COMM_BUILD']:
+    if not CONFIG['LIBXUL_SDK']:
+        include('/toolkit/toolkit.mozbuild')
+    if CONFIG['MOZ_EXTENSIONS']:
+        add_tier_dir('app', 'toolkit/extensions')

--- a/im/build.mk
+++ b/im/build.mk
@@ -1,25 +1,13 @@
 # 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/.
 
-ifndef COMM_BUILD
-
-ifndef LIBXUL_SDK
-include $(topsrcdir)/toolkit/toolkit-tiers.mk
-endif
-
-TIERS += app
-
-ifdef MOZ_EXTENSIONS
-tier_app_dirs += extensions
-endif
-
-else
+ifdef COMM_BUILD
 
 ifndef MOZILLA_DIR
 MOZILLA_DIR = $(MOZILLA_SRCDIR)
 endif
 include $(MOZILLA_SRCDIR)/toolkit/mozapps/installer/package-name.mk
 
 TIERS += app