Examine source code of Close Tab Button

{
  "manifest_version": 3,
  "name": "Close Tab Button",
  "version": "1.0",
  "description": "Close the active tab.",
  "permissions": [
    "tabs"
  ],
  "action": {
    "default_title": "Close tab",
    "default_icon": {
      "16": "icon.png",
      "32": "icon.png"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "icons": {
    "16": "icon.png",
    "32": "icon.png",
    "128": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "close-tab@david.local"
    }
  }
}