diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index b7dd298c4ffa1b95531e3646d57a52e524fcf801..da23bcd2a8b4b49ce07f4602f5e624bf4bfbfc8d 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -282,6 +282,7 @@ for var in (
     "DLL_SUFFIX",
     "DEBUG_JS_MODULES",
     "OMNIJAR_NAME",
+    "ZEN_FIREFOX_VERSION"
 ):
     DEFINES[var] = CONFIG[var] or ""
 
