Examine source code of VKMeOnlineUsers

{
  "manifest_version": 2,
  "name": "VKMeOnlineUsers",
  "version": "1.0",
  "description": "Will add the ability to view online users of VKontakte when hidden mode is activated. Update every 5 seconds.",
  "applications": {
    "gecko": {
      "id": "KseniMailwow@Gmail.com",
      "strict_min_version": "48.0"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://vk.com/*"
      ],
      "js": [
        "VkUpdate.js"
      ]
    }
  ]
}