Examine source code of Inevitable Calendar

{
  "manifest_version": 2,
  "name": "Inevitable Death Countdown",
  "version": "1.0",
  "description": "Visualize your inevitable death countdown",
  "chrome_url_overrides": {
    "newtab": "newtab.html"
  },
  "permissions": [
    "storage"
  ],
  "icons": {
    "48": "icons/inevitable-48.jpg",
    "96": "icons/inevitable-96.jpg"
  },
  "applications": {
    "gecko": {
      "id": "inevitable@stuckinforloop.com",
      "strict_min_version": "58.0"
    }
  }
}