diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index a17e15c74c8e627cf805d1f026438759af7e4a97..fc6e1e6b2711e474d65a688b2481b7a26cad2a71 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -280,6 +280,7 @@ for var in (
     "DLL_SUFFIX",
     "DEBUG_JS_MODULES",
     "OMNIJAR_NAME",
+    "ZEN_FIREFOX_VERSION"
 ):
     DEFINES[var] = CONFIG[var] or ""