Examine source code of GIS Custom Startpage

{
  "manifest_version": 3,
  "name": "Custom Startpage",
  "version": "1.4",
  "description": "A custom start page for Firefox and Chrome",
  "chrome_url_overrides": {
    "newtab": "startpage.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "startpage@yourdomain.com",
      "strict_min_version": "109.0"
    }
  },
  "permissions": [],
  "icons": {
    "48": "icon.png"
  }
}