diff --git a/browser/base/content/browser-init.js b/browser/base/content/browser-init.js
index 2f3f46d8d82dc90ebca8d64ace9d6bf1ee127937..163eb027b8cca1d43af24fff762e951e6f1030fc 100644
--- a/browser/base/content/browser-init.js
+++ b/browser/base/content/browser-init.js
@@ -279,6 +279,7 @@ var gBrowserInit = {
   },
 
   onLoad() {
+    Services.scriptloader.loadSubScript("chrome://browser/content/zenThemeModifier.js", window);
     gBrowser.addEventListener("DOMUpdateBlockedPopups", e =>
       PopupAndRedirectBlockerObserver.handleEvent(e)
     );