Examine source code of Left Tab Focus on Close

{
  "manifest_version": 3,
  "name": "Left Tab Focus on Close",
  "version": "1.1.1",
  "description": "Activates the tab to the left when the current Firefox tab is closed.",
  "permissions": [
    "storage",
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "left-tab-focus-on-close@example.local",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  }
}