fix(manifest): another attempt to fix firefox

manifest/firefox.json

@@ -1,6 +1,5 @@
 {
   "manifest_version": 3,
-  "id": "muetab@muetab.com",
   "name": "Mue",
   "description": "Fast, open and free-to-use new tab page for modern browsers.",
   "version": "7.1.2",
@@ -18,5 +17,10 @@
   },
   "chrome_settings_overrides": {
     "homepage": "index.html"
+  },
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "muetab@muetab.com"
+    }
   }
 }