Examine source code of Viewport

{
  "manifest_version": 2,
  "name": "Viewport",
  "version": "1.2resigned1",
  "description": "Displays the current viewport size when resizing the browser window.",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "viewport.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{13612168-7d1f-4ccf-a787-7e4b4e00ac70}"
    }
  }
}