Examine source code of Custom Start Tab

{
  "manifest_version": 3,
  "name": "Custom Start Tab",
  "description": "Custom start tab by ShamHyper",
  "version": "1.12.1",
  "action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "chrome_url_overrides": {
    "newtab": "index.html"
  },
  "icons": {
    "16": "icon.png",
    "48": "icon.png",
    "128": "icon.png"
  },
  "permissions": [
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "customchrometab@firefox-id"
    }
  }
}