Examine source code of Font Inspector

{
  "manifest_version": 3,
  "name": "Font Inspector",
  "version": "1.0",
  "description": "Shows the font used on the current website",
  "permissions": [
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "font-inspector@example.com"
    }
  }
}