Examine source code of Happy Swing

{
  "name": "Happy Swing",
  "version": "3.0",
  "manifest_version": 2,
  "description": "you can play game Happy Swing directly from the sidebar!",
  "permissions": [
    "tabs",
    "<all_urls>"
  ],
  "browser_action": {
    "default_title": "Side Games - play side panel mini games",
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "sidebar_action": {
    "default_title": "Side Games",
    "default_panel": "sidebar.html",
    "default_icon": "icon.png"
  },
  "applications": {},
  "browser_specific_settings": {
    "gecko": {
      "id": "{86976773-7f4e-4458-a564-83ec3a97dc02}"
    }
  }
}