Examine source code of Square Crop Preview

{
  "manifest_version": 2,
  "name": "Square Crop Preview",
  "version": "1.0",
  "description": "Show a square crop preview when hovering over images. Preview only.",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "square-crop-preview@amo.local",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  }
}