Examine source code of Append To Domain

{
  "description": "Appends custom string to current domain name",
  "manifest_version": 2,
  "name": "Append To Domain",
  "version": "1.2resigned1",
  "background": {
    "scripts": [
      "append.js"
    ]
  },
  "options_ui": {
    "page": "options.html"
  },
  "permissions": [
    "storage",
    "tabs"
  ],
  "applications": {
    "gecko": {
      "id": "append2domain@example.com"
    }
  },
  "browser_action": {
    "default_icon": {
      "16": "icons/icon-16.png",
      "32": "icons/icon-32.png",
      "64": "icons/icon-64.png"
    }
  }
}