Examine source code of Tracking Timer

{
  "manifest_version": 2,
  "name": "Tracking Timer",
  "description": "Use timers to track time fast and easily.",
  "version": "1.5",
  "icons": {
    "48": "assets/app-icon.svg",
    "96": "assets/app-icon.svg"
  },
  "browser_action": {
    "default_icon": "assets/app-icon.svg",
    "default_popup": "index.html",
    "default_title": "Tracking Timer"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "storage",
    "unlimitedStorage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "TrackingTimer@RT"
    }
  }
}