Examine source code of Danielification

{
  "manifest_version": 2,
  "version": "0.2.3",
  "name": "Danielification",
  "content_scripts": [
    {
      "matches": [
        "https://www.youtube.com/*"
      ],
      "js": [
        "main.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{6f045a03-c9d0-4e56-ab00-0e3159def918}"
    }
  }
}