Examine source code of workTimer

{
  "description": "Timer for task followup while working.",
  "manifest_version": 2,
  "name": "workTimer",
  "version": "1.0",
  "homepage_url": "https://github.com/depuits/workTimer",
  "icons": {
    "48": "icons/icon-48.png"
  },
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_action": {
    "default_icon": "icons/icon-32.png",
    "theme_icons": [
      {
        "light": "icons/icon-32-light.png",
        "dark": "icons/icon-32.png",
        "size": 32
      }
    ],
    "default_title": "workTimer",
    "default_popup": "popup/worktimer.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{c480c451-fa3c-4fac-b33f-d5cfedf751c5}"
    }
  }
}