Examine source code of YouTube Thumbnail Grabber

{
  "manifest_version": 2,
  "name": "YouTube Thumbnail Grabber",
  "version": "1.2.1",
  "description": "Adds a Thumbnail of the currently watched Video.",
  "icons": {
    "48": "icons/Youtube-Thumbnail-Grabber-48.png",
    "64": "icons/Youtube-Thumbnail-Grabber-64.png",
    "128": "icons/Youtube-Thumbnail-Grabber-128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/watch?v=*"
      ],
      "js": [
        "jquery.3.3.1.min.js",
        "Seventh.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{514dee28-313b-472e-a496-7cfeaef3adb1}"
    }
  }
}