Examine source code of Element Snapper

{
  "manifest_version": 2,
  "name": "Element Snapper",
  "version": "1.0.2",
  "description": "Select multiple HTML elements on any webpage and save each as a separate PNG screenshot. No html2canvas — uses native Firefox screenshot API for pixel-perfect results.",
  "author": "ASRRCRM",
  "homepage_url": "https://asrrcrm.com",
  "permissions": [
    "activeTab",
    "tabs",
    "downloads",
    "<all_urls>"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "element-snapper@asrrcrm.com",
      "strict_min_version": "86.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  },
  "browser_action": {
    "default_icon": {
      "32": "icons/icon32.png",
      "48": "icons/icon48.png"
    },
    "default_title": "Element Snapper by ASRRCRM",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "content.css"
      ],
      "run_at": "document_idle"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "icons": {
    "32": "icons/icon32.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  }
}