Examine source code of Countdown Timer

{
  "manifest_version": 3,
  "name": "Countdown Timer",
  "version": "1.0",
  "description": "A simple countdown timer for your browser.",
  "permissions": [],
  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "48": "icon.png"
    }
  },
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "countdown-timer@example.com"
    }
  }
}