diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index e6a9634bc5ae26a5bc82402a8a0f37e1fedc27ca..e27cab2a984b2d09033bc956df90519e01f683d7 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -277,6 +277,7 @@ for var in (
     "DLL_SUFFIX",
     "DEBUG_JS_MODULES",
     "OMNIJAR_NAME",
+    "ZEN_FIREFOX_VERSION"
 ):
     DEFINES[var] = CONFIG[var] or ""
 
