Examine source code of Tab Reloader – Page Timer

{
  "manifest_version": 2,
  "name": "Tab Reloader — Interval & Page Timer",
  "version": "3.2",
  "description": "Auto-reload any tab on a custom interval with a draggable on-page countdown timer. Control everything from the popup or directly from the page.",
  "permissions": [
    "tabs",
    "storage",
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "tab-reloader@liampl",
      "strict_min_version": "140.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "overlay.css"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "Tab Reloader"
  },
  "icons": {
    "48": "icons/icon48.png",
    "96": "icons/icon96.png"
  }
}