diff --git a/mozilla/calendar/lightning/Makefile.in b/mozilla/calendar/lightning/Makefile.in
index bfee4041d09..3582e72a7b0 100644
--- a/mozilla/calendar/lightning/Makefile.in
+++ b/mozilla/calendar/lightning/Makefile.in
@@ -21,6 +21,7 @@
 #   Mike Shaver <shaver@off.net>
 #   Matthew Willis <lilmatt@mozilla.com>
 #   Philipp Kewisch <mozilla@kewis.ch>
+#   Dan Mosedale <dmose@mozilla.org>
 #
 # Alternatively, the contents of this file may be used under the terms of
 # either the GNU General Public License Version 2 or later (the "GPL"), or
@@ -60,9 +61,13 @@ ifdef LIGHTNING_FIREFOX
 FIREFOX_VERSION := $(shell cat $(MOZILLA_SRCDIR)/browser/config/version.txt)
 endif
 
-# don't install by default, it causes problems on tinderboxes
-# (for example, see bug 406441)
-ifdef INSTALL_LIGHTNING
+# include config.mk here so myconfig or app-config can set DISABLE_LIGHTNING_INSTALL
+include $(topsrcdir)/config/config.mk
+
+# installing lightning in a thunderbird build causes problems on tinderboxes
+# (see bug 406441 and bug 440017), so we need to provide a hook for the
+# tinderboxen to disable that.
+ifndef DISABLE_LIGHTNING_INSTALL
 ifndef MOZ_SUNBIRD
 # install Lightning as a global extension in dist/bin/extensions/
 INSTALL_EXTENSION_ID   = {e2fda1a4-762b-4020-b5ad-a41df1933103}