Examine source code of Loading Song

{
  "manifest_version": 2,
  "name": "Loading Song",
  "version": "1.0",
  "description": "Plays a short melodic phrase while a page is loading and finishes with a resolving chord when load completes.",
  "icons": {
    "48": "icons/icon-48.png",
    "128": "icons/icon-128.png"
  },
  "permissions": [
    "tabs",
    "storage",
    "notifications",
    "activeTab",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "browser_action": {
    "default_icon": "icons/icon-48.png",
    "default_title": "Loading Song",
    "default_popup": "popup.html"
  },
  "applications": {
    "gecko": {
      "id": "loadingsong@example.com"
    }
  }
}