diff --git a/src/background.thunderbird.js b/src/background.thunderbird.js
index 7b8e29c..ab0c5c0 100644
--- a/src/background.thunderbird.js
+++ b/src/background.thunderbird.js
@@ -1,3 +1,4 @@
+/* global messenger */
 import browser from "webextension-polyfill";
 import {
   MSG_FETCH,
@@ -251,4 +252,4 @@ async function registerMsgDisplayScript() {
 			js: [{file: "/content.js"}]
 		});
 	}
-registerMsgDisplayScript();
\ No newline at end of file
+registerMsgDisplayScript();