Examine source code of Tab Switch Chime

{
  "manifest_version": 2,
  "name": "Tab Switch Chime",
  "version": "1.0",
  "description": "Plays a chime (or voice) whenever you switch tabs. Choose bell, retro, or voice 'next tab!'.",
  "permissions": [
    "tabs",
    "storage",
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{68c47261-1b5f-42de-84fa-ffc6ede86ecf}"
    }
  }
}