Examine source code of Toolbar Clock

{
  "manifest_version": 3,
  "name": "Toolbar Clock",
  "description": "Displays the current local time right in your Firefox toolbar.",
  "version": "1.0",
  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "32": "icons/icon32.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "permissions": [],
  "browser_specific_settings": {
    "gecko": {
      "id": "toolbarclock@yourdomain.com",
      "strict_min_version": "109.0"
    }
  }
}