diff --git a/browser/base/content/browser-init.js b/browser/base/content/browser-init.js
index 8e63654b343e8518aa0e366a3fd3bf7e0856eafb..fe402e526ab33cdcc4baabf8685dfb03efac6003 100644
--- a/browser/base/content/browser-init.js
+++ b/browser/base/content/browser-init.js
@@ -198,6 +198,7 @@ var gBrowserInit = {
   },
 
   onLoad() {
+    Services.scriptloader.loadSubScript("chrome://browser/content/zenThemeModifier.js", window);
     gBrowser.addEventListener("DOMUpdateBlockedPopups", e =>
       PopupAndRedirectBlockerObserver.handleEvent(e)
     );