Examine source code of Print this page

{
  "name": "Print this page",
  "description": "Adds a print button to the browser.",
  "version": "3.1",
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_title": "Print this page",
    "default_icon": "print_16x16.png"
  },
  "manifest_version": 2,
  "browser_specific_settings": {
    "gecko": {
      "id": "{c91b1e70-97c6-4787-99dd-d834e84e5cd8}"
    }
  }
}