Examine source code of ReplyCounter

{
  "manifest_version": 2,
  "name": "ReplyCounter",
  "version": "1.1",
  "permissions": [
    "storage"
  ],
  "web_accessible_resources": [
    "replycounter.html"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "@replycounter",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "description": "Counts twitter replies sent per day.",
  "icons": {
    "48": "48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.x.com/*"
      ],
      "js": [
        "replycounter.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  }
}