diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index 4c45be797e2cca88d302f98e765545b13da3f0dc..31f2694a8cfb73c4f31b819e4362b46bdb22faef 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 ""
 
