Examine source code of Countdown Stack

{
  "manifest_version": 2,
  "name": "Countdown Stack",
  "description": "Stack up as many countdowns as you need and watch the nearest one in the toolbar. Durations accept 90, 1:30, 5m or 1h 20m, and paused timers keep exactly what was left.",
  "version": "3.1.5.9",
  "browser_specific_settings": {
    "gecko": {
      "id": "{566cdfbf-2818-4118-978a-a96f20441401}",
      "strict_min_version": "101.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "icons": {
    "32": "graphics/icon-32.png",
    "48": "graphics/icon-48.png",
    "64": "graphics/icon-64.png",
    "96": "graphics/icon-96.png",
    "128": "graphics/icon-128.png"
  },
  "browser_action": {
    "default_title": "Countdown Stack",
    "default_popup": "popup.html",
    "default_icon": {
      "32": "graphics/icon-32.png",
      "48": "graphics/icon-48.png",
      "64": "graphics/icon-64.png"
    }
  },
  "permissions": [
    "storage",
    "notifications",
    "alarms"
  ],
  "background": {
    "scripts": [
      "app/core.js",
      "app/background.js"
    ],
    "persistent": true
  }
}