diff --git a/content/install-hook-userstylesworld.js b/content/install-hook-userstylesworld.js
index 4552f75c..66801e12 100644
--- a/content/install-hook-userstylesworld.js
+++ b/content/install-hook-userstylesworld.js
@@ -29,9 +29,9 @@
     },
   });
 
-  window.addEventListener('message', ({data, source}) => {
-    // Accepting events only from this page.
-    if (data && source === window) {
+  window.addEventListener('message', ({data, source, origin}) => {
+    // Some browser don't reveal `source` to extensions e.g. Firefox
+    if (data && (source ? source === window : origin === ORIGIN)) {
       const fn = HANDLERS[data.type];
       if (fn) fn(data);
     }