Examine source code of YouTube Focus Mode i

{
  "manifest_version": 2,
  "name": "YouTube Focus Mode",
  "version": "1.0",
  "description": "Hide distracting elements on YouTube to help you stay focused.",
  "icons": {
    "48": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://www.youtube.com/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "permissions": [
    "tabs"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{2d57ff62-8a49-422d-b44c-1d2114b88904}"
    }
  }
}