Examine source code of Focus Mode

{
  "manifest_version": 2,
  "name": "Focus Mode",
  "version": "0.1",
  "description": "Hide youtube recomandation, comments, next videos etc.",
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "js": [
        "focus-mode.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{f28efdc9-c364-4234-b460-a7e70b8b5e83}"
    }
  }
}