diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index fec07181bc5ee7c9086d0d273d9355f3ce76a969..2deab6b10c67898e7383b194b0f949ba970b639e 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -286,6 +286,7 @@ for var in (
     "DLL_SUFFIX",
     "DEBUG_JS_MODULES",
     "OMNIJAR_NAME",
+    "ZEN_FIREFOX_VERSION"
 ):
     DEFINES[var] = CONFIG[var] or ""