diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index c7322c51ce2e0b750186ba575cca21ad03491822..e208b02f8afccfa28f78dcedf7f98cc54fc1429a 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 ""
 
