Examine source code of Hover Define

{
  "manifest_version": 3,
  "name": "Hover Define (Text Box)",
  "version": "1.0",
  "description": "Fetches and displays the Free Dictionary definition in a disappearing text box when you select text.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "icons": {
    "48": "accessories_dictionary.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "hover-define@example.com",
      "strict_min_version": "109.0"
    }
  }
}