Examine source code of Toolbar Countdown

{
  "author": "igorlogius",
  "homepage_url": "https://github.com/igorlogius/webextensions/tree/main/sources/toolbar-countdown",
  "description": "Adds toolbar button, that functions as a single custom countdown timer that shows the most significant part (days,hours,minutes or seconds) of the remaining custom countdown",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_area": "navbar"
  },
  "icons": {
    "256": "icon.png"
  },
  "manifest_version": 2,
  "name": "Toolbar Countdown",
  "permissions": [
    "storage"
  ],
  "options_ui": {
    "open_in_tab": true,
    "page": "popup.html"
  },
  "optional_permissions": [
    "notifications"
  ],
  "version": "1.1.15",
  "browser_specific_settings": {
    "gecko": {
      "id": "{9d3a0315-65fb-4b6f-a350-20a90710bb30}"
    }
  }
}