Examine source code of YouTube 4 Columns

{
  "manifest_version": 2,
  "name": "YouTube 4 Columns",
  "version": "1.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "@YouTube-4-Columns",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "description": "Force YouTube to show 4 videos per row",
  "icons": {
    "128": "youtube-icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_start"
    }
  ]
}