Examine source code of YouTube Sync

{
  "manifest_version": 2,
  "name": "YouTube Sync",
  "version": "1.1",
  "description": "Sync YouTube playback between devices over local network",
  "permissions": [
    "storage",
    "tabs",
    "*://*.youtube.com/*",
    "*://*.rutube.ru/*",
    "http://192.168.1.122:5000/*"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*",
        "*://*.rutube.ru/*"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "icons": {
    "48": "icon48.png",
    "96": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "yt-sync@zabeiua.github.io",
      "data_collection_permissions": {
        "required": [
          "personallyIdentifyingInfo"
        ]
      }
    },
    "gecko_android": {
      "strict_min_version": "142.0"
    }
  }
}