Examine source code of Custom newtab browser

{
  "manifest_version": 2,
  "name": "Custom newtab browser",
  "version": "1.0",
  "description": "Custom newtab for your browser",
  "icons": {
    "128": "icon128.png"
  },
  "permissions": [
    "storage",
    "geolocation"
  ],
  "chrome_url_overrides": {
    "newtab": "index.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{bf9c8f36-192d-49f0-a1a8-fc9d9fab4ee6}"
    }
  }
}