Examine source code of Page Load Alert

{
  "manifest_version": 2,
  "name": "Page Load Alert",
  "version": "1",
  "description": "Adds an alert sound when a page has finished loading.",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "js/alert.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "icons": {
    "48": "icons/bell.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{19d977da-17d7-494d-bc55-892b330c8b43}"
    }
  }
}