feat: add browser-specific settings for Firefox extension

Author: medyo

public/firefox.manifest.json

@@ -1,5 +1,10 @@
 {
   "background": {
     "scripts": ["background.js"]
+  },
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "f8793186-e9da-4332-aa1e-dc3d9f7bb04c"
+    }
   }
 }