diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index 37f4a1adbfbcddaa5dd72a02befe67901d89d5a8..c8360e3e8d183dbd108ba9f953013e625cadc6f5 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -290,6 +290,7 @@ for var in (
     "DLL_SUFFIX",
     "DEBUG_JS_MODULES",
     "OMNIJAR_NAME",
+    "ZEN_FIREFOX_VERSION"
 ):
     DEFINES[var] = CONFIG[var] or ""
 
