Examine source code of Mirror extension!

{
  "manifest_version": 2,
  "name": "Mirror extension!",
  "description": "The mirror extension mirrors web pages!",
  "version": "1.1.0",
  "icons": {
    "16": "images/icon16.png",
    "32": "images/icon32.png",
    "48": "images/icon48.png",
    "128": "images/icon128.png"
  },
  "permissions": [
    "activeTab",
    "scripting"
  ],
  "background": {
    "scripts": [
      "service.js"
    ]
  },
  "browser_action": {
    "default_icon": "images/icon32.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{c6112968-77e0-4c75-9964-a8aeb913ecf7}"
    }
  }
}