Examine source code of EstateSales → Calendar

{
  "manifest_version": 3,
  "name": "EstateSales → Calendar",
  "version": "1.1",
  "description": "Adds an 'Add to Calendar' button to EstateSales.NET sale pages.",
  "permissions": [
    "storage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "estatesales-calendar@extension",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      },
      "strict_min_version": "141.0"
    }
  },
  "options_ui": {
    "page": "options.html",
    "open_in_tab": false
  },
  "content_scripts": [
    {
      "matches": [
        "*://www.estatesales.net/*"
      ],
      "js": [
        "compat.js",
        "content.js"
      ],
      "css": [
        "styles.css"
      ],
      "run_at": "document_idle"
    }
  ],
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon128.png"
  }
}