Examine source code of WhatsStats

{
  "manifest_version": 2,
  "name": "WhatsStat",
  "version": "0.1",
  "description": "stats for whatsapp",
  "icons": {
    "48": "icons/beasts-48.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/beasts-48.png",
    "default_title": "WhatsStat",
    "default_popup": "popup/main_dialog.html"
  },
  "_content_scripts": [
    {
      "matches": [
        "*://web.whatsapp.com/*",
        "*://web.whatsapp.com/",
        "https://web.whatsapp.com/",
        "https://www.google.com/"
      ],
      "js": [
        "main.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{721f567f-f060-47b9-954b-708c39302b75}"
    }
  }
}