diff --git a/mozilla/layout/tools/reftest/Makefile.in b/mozilla/layout/tools/reftest/Makefile.in
index 88b7072b30e..85f11aa9663 100644
--- a/mozilla/layout/tools/reftest/Makefile.in
+++ b/mozilla/layout/tools/reftest/Makefile.in
@@ -118,7 +118,7 @@ autoconf.js: $(DEPTH)/config/autoconf.mk Makefile
 	# strip comments, escape \ and ", use only assignment lines that
 	# don't end in backslashes and don't have variables, and then
 	# convert to JS properties
-	cat $< | perl -p -e 's/#.*//;s,\\,\\\\,g;s,",\\",g' | grep '=' | grep -v '\\$$' | grep -v '\$$' | perl -p -e 's/[ \t]*:?= *(.*)/: "\1",/' >> $@
+	cat $< | egrep -v "^export " | perl -p -e 's/#.*//;s,\\,\\\\,g;s,",\\",g' | grep '=' | grep -v '\\$$' | grep -v '\$$' | perl -p -e 's/[ \t]*:?= *(.*)/: "\1",/' >> $@
 	echo 'dummy_var: null' >> $@ # to avoid trailing comma
 	echo '};' >> $@