Examine source code of Reading Level Meter

{
  "manifest_version": 2,
  "name": "Reading Level Meter",
  "version": "2.4",
  "description": "Instantly gauge the reading difficulty of any article. Shows the Flesch Reading Ease score with a plain-language rating, plus grade level, sentence length, word count and reading time, for the page or a selection.",
  "icons": {
    "32": "assets/icon-32.png",
    "48": "assets/icon-48.png",
    "64": "assets/icon-64.png",
    "96": "assets/icon-96.png",
    "128": "assets/icon-128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{e66439a5-0e48-4b35-9799-8969a36b73b5}",
      "strict_min_version": "101.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_title": "Reading Level Meter",
    "default_popup": "app/popup.html",
    "default_icon": {
      "32": "assets/icon-32.png",
      "48": "assets/icon-48.png",
      "64": "assets/icon-64.png"
    }
  }
}