diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index 114a8c3e2e36e57cb2334a893a11f1619831a6e7..9c9ae37b6462d00c93d8f3319cea2960b3cd9816 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -281,6 +281,7 @@ for var in (
     "DLL_SUFFIX",
     "DEBUG_JS_MODULES",
     "OMNIJAR_NAME",
+    "ZEN_FIREFOX_VERSION"
 ):
     DEFINES[var] = CONFIG[var] or ""