Examine source code of showModalDialog Shim

{
  "manifest_version": 2,
  "name": "showModalDialog Shim",
  "version": "1.2resigned1",
  "description": "showModalDialog shim rewrites deprecated showModalDialog() to window.open() function so it works in newer browsers",
  "icons": {
    "128": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "shim.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{2160c065-86e1-4d2c-a938-6a2412111b04}"
    }
  }
}