diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index e43870b10c5b35165fd21b48065980060104dc52..71e3db5618c834bc4ff9d481b1e667f2b02b183e 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 ""