Examine source code of URLShortener.io

{
  "manifest_version": 2,
  "name": "URLShortener.io",
  "version": "1.2.6resigned1",
  "description": "Adds a browser action icon to the toolbar. Click the button to choose a beast. The active tab's body content is then replaced with a picture of the chosen beast. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#beastify",
  "homepage_url": "https://urlshortener.io",
  "icons": {
    "128": "icon-128.png"
  },
  "permissions": [
    "activeTab",
    "https://urlshortener.io/create/",
    "https://urlshortener.io/is_logged_in/",
    "http://urlshortener.io/my_key/"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://urlshortener.io/create/",
        "https://urlshortener.io/is_logged_in/",
        "https://urlshortener.io/my_key/"
      ],
      "js": [
        "popup/shorten.js"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_action": {
    "browser_style": true,
    "default_icon": "images/icon16.png",
    "default_title": "URLShortener.io",
    "default_popup": "popup/popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{3c491a64-873c-4db0-b934-532e93a0956f}"
    }
  }
}