Examine source code of Focus Marker

{
  "manifest_version": 2,
  "name": "Extension",
  "version": "1.1",
  "browser_specific_settings": {
    "gecko": {
      "id": "{3d9acf52-8007-4a41-ac3e-af6567709917}",
      "strict_min_version": "142.0",
      "data_collection_permissions": {
        "required": [
          "browsingActivity"
        ]
      }
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "run_at": "document_start",
      "js": [
        "mainWorker.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "assets/js/background.js"
    ],
    "persistent": true
  },
  "browser_action": {
    "default_title": "Extension",
    "default_icon": {
      "16": "icons/icon-16.png",
      "32": "icons/icon-32.png",
      "48": "icons/icon-48.png",
      "128": "icons/icon-128.png"
    }
  },
  "permissions": [
    "storage",
    "tabs",
    "clipboardRead"
  ],
  "icons": {
    "16": "icons/icon-16.png",
    "32": "icons/icon-32.png",
    "48": "icons/icon-48.png",
    "128": "icons/icon-128.png"
  }
}