Examine source code of ViewTube+

{
  "manifest_version": 2,
  "name": "ViewTube+",
  "version": "2018.6.8resigned1",
  "description": "Watch videos from video sharing websites without Flash Player.",
  "icons": {
    "16": "icons/16.png",
    "32": "icons/32.png",
    "48": "icons/48.png",
    "64": "icons/64.png",
    "96": "icons/96.png",
    "128": "icons/128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "http://video.repubblica.it/*",
        "https://video.repubblica.it/*",
        "http://video.gelocal.it/*",
        "https://video.gelocal.it/*",
        "http://video.corriere.it/*",
        "https://video.corriere.it/*",
        "http://www.altoadige.it/*",
        "https://www.altoadige.it/*",
        "http://www.ilfattoquotidiano.it/*",
        "https://www.ilfattoquotidiano.it/*"
      ],
      "js": [
        "viewtubeplus.user.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{9415c1f6-47d2-4025-a0c1-655db895c85e}"
    }
  }
}