Examine source code of Recently Closed

{
  "manifest_version": 2,
  "applications": {
    "gecko": {
      "id": "recentlyclosed@webExtension.arielg.il",
      "strict_min_version": "60.0"
    }
  },
  "name": "RecentlyClosed",
  "version": "1.0",
  "description": "Reopen recently closed tabs or windows.",
  "author": "arielg",
  "permissions": [
    "sessions",
    "menus",
    "tabs"
  ],
  "icons": {
    "16": "icons/recentlyClosed.svg",
    "32": "icons/recentlyClosed.svg",
    "48": "icons/recentlyClosed.svg"
  },
  "browser_action": {
    "browser_style": true,
    "default_title": "Reopen Recently Closed",
    "default_icon": {
      "16": "icons/recentlyClosed.svg",
      "32": "icons/recentlyClosed.svg",
      "48": "icons/recentlyClosed.svg"
    }
  },
  "commands": {
    "kb-reopen-recently-closed": {
      "suggested_key": {
        "default": "Ctrl+Alt+R"
      },
      "description": "Reopen Recently Closed"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  }
}