Examine source code of Undo Closed Tabs

{
  "manifest_version": 2,
  "name": "Undo Tab",
  "version": "1.0",
  "description": "Instantly restore your last closed tab with a single click",
  "permissions": [
    "sessions"
  ],
  "browser_action": {
    "default_icon": {
      "16": "icon.png"
    }
  },
  "icons": {
    "16": "icon.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{957a27a9-aa3d-4588-b212-dba5e3a6c7cd}"
    }
  }
}