Examine source code of Old Facebook Layout Restorer

{
  "description": "Reverts Facebook to its classic pre-September 2020 layout by modifying your browser's user-agent string to an unsupported version, bringing back the familiar interface",
  "manifest_version": 2,
  "name": "Old Facebook Layout Restorer",
  "author": "Matt Kruse",
  "version": "2.0",
  "homepage_url": "https://OldLayout.com",
  "icons": {
    "16": "icon-16.png",
    "32": "icon-32.png",
    "48": "icon-48.png",
    "128": "icon-128.png"
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "*://*.facebook.com/*"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.facebook.com/*"
      ],
      "css": [
        "style.css"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_action": {
    "default_icon": {
      "16": "icon-16.png",
      "19": "icon-19.png",
      "32": "icon-32.png",
      "38": "icon-38.png",
      "48": "icon-48.png"
    },
    "default_title": "Old Layout for Facebook",
    "default_popup": "./options.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{94f22f3e-9135-4895-8c87-96f255d80e62}"
    }
  }
}