diff --git a/src/extension/manifest-firefox.json b/src/extension/manifest-firefox.json
index 7221b8f2..e61f298c 100644
--- a/src/extension/manifest-firefox.json
+++ b/src/extension/manifest-firefox.json
@@ -1,4 +1,9 @@
 {
+  "background": {
+    "scripts": ["background/index.js"],
+    "type": "module"
+  },
+
   "browser_specific_settings": {
     "gecko": {
       "id": "stylebot@stylebot.dev"