Examine source code of Close Duplicate Sites

{
  "manifest_version": 2,
  "name": "Close Duplicate Sites",
  "version": "1.1.1",
  "description": "Closes duplicate tabs by base domain (eTLD+1), keeping the active tab.",
  "browser_specific_settings": {
    "gecko": {
      "id": "{454c608e-0d1f-42c6-9738-bda49d512b18}",
      "strict_min_version": "140.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "permissions": [
    "tabs",
    "menus",
    "storage",
    "alarms",
    "https://publicsuffix.org/list/public_suffix_list.dat"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_title": "Close duplicate sites (keep active)",
    "default_icon": "icons/duplicate.svg"
  },
  "icons": {
    "48": "icons/duplicate.svg"
  }
}