Examine source code of Notification Beatbox

{
  "manifest_version": 2,
  "name": "Notification Beatbox",
  "version": "1.0",
  "description": "Replaces page-created notifications with fun beatbox/instrument sounds like 'boom-tsk!'.",
  "permissions": [
    "storage",
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{f6d7afe7-9b88-40cf-9961-b5cb24724097}"
    }
  }
}