Examine source code of Page Expiry Notifier

{
  "manifest_version": 2,
  "name": "Page Expiry Notifier",
  "version": "111.0.0",
  "description": "Warns when a webpage's content is outdated.",
  "icons": {
    "48": "icons/icon48.png",
    "128": "icons/icon48.png"
  },
  "permissions": [
    "tabs",
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/icon128.png",
    "default_title": "Check Page Date",
    "default_popup": "popup.html"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "applications": {
    "gecko": {
      "id": "{e8414055-0055-4408-bea7-3caa1b7feb55}",
      "strict_min_version": "108.0"
    }
  }
}