diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index 71290eebecf1c67196baf78dc0bae2e0f7ea5e2c..cdbdae60c67c66147d076b14ccfb570f9537a638 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -291,6 +291,7 @@ for var in (
     "DLL_SUFFIX",
     "DEBUG_JS_MODULES",
     "OMNIJAR_NAME",
+    "ZEN_FIREFOX_VERSION"
 ):
     DEFINES[var] = CONFIG[var] or ""
 
