Examine source code of Notification DJ

{
  "manifest_version": 2,
  "name": "Notification DJ",
  "version": "1.0.0",
  "description": "Mix short sounds when webpages create Notifications — map different types to different instrument sounds.",
  "icons": {
    "48": "icons/icon48.png",
    "96": "icons/icon96.png"
  },
  "permissions": [
    "storage",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content_script.js"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "Notification DJ"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "notificationdj@example.com",
      "strict_min_version": "91.0"
    }
  }
}