Examine source code of DetailZoom

{
  "manifest_version": 2,
  "name": "DetailZoom",
  "version": "1.0",
  "description": "Show hidden details when zooming images: pixel grid, contrast map, dominant color hotspots.",
  "icons": {},
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "detail-zoom.js"
      ],
      "run_at": "document_end"
    }
  ],
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{3e668952-7593-4e44-a3f3-98ad35843434}"
    }
  }
}