Examine source code of Text Character Counter

{
  "manifest_version": 2,
  "name": "Text Character Counter",
  "version": "1.0",
  "description": "選択テキストの文字数をカウントします",
  "browser_specific_settings": {
    "gecko": {
      "id": "text-character-counter@your-domain.com",
      "strict_min_version": "140.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    },
    "gecko_android": {
      "strict_min_version": "142.0"
    }
  },
  "permissions": [
    "activeTab",
    "tabs"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "48": "icons/icon.svg"
    },
    "default_title": "Text Character Counter"
  }
}