Examine source code of Close discarded tabs

{
  "manifest_version": 2,
  "name": "Close Discarded Tabs",
  "version": "1.2.3",
  "description": "Adds a button to the toolbar that closes discarded tabs when clicked",
  "homepage_url": "https://github.com/scriptsmith/cdt",
  "icons": {
    "48": "icons/trash-48.png"
  },
  "permissions": [
    "tabs",
    "storage"
  ],
  "browser_action": {
    "default_icon": "icons/trash-48.png",
    "default_title": "Close Discarded Tabs"
  },
  "background": {
    "scripts": [
      "main.js"
    ]
  },
  "options_ui": {
    "page": "options.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{0587456e-e01b-472d-9109-81ca4a25367d}"
    }
  }
}