Examine source code of Close this tab

{
  "manifest_version": 2,
  "name": "Close this tab",
  "version": "1.0",
  "description": "Add a \"Close this tab\" button, with similar style as the \"Open a new tab\" button",
  "icons": {
    "48": "icon-white.svg",
    "96": "icon-white.svg"
  },
  "background": {
    "scripts": [
      "index.js"
    ]
  },
  "browser_action": {
    "default_area": "navbar",
    "default_title": "Close this tab (Ctrl+W)",
    "default_icon": {
      "16": "icon-white.svg",
      "32": "icon-white.svg"
    },
    "theme_icons": [
      {
        "light": "icon-white.svg",
        "dark": "icon-black.svg",
        "size": 16
      },
      {
        "light": "icon-white.svg",
        "dark": "icon-black.svg",
        "size": 32
      }
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{ff971f80-4a75-4f4b-a3ee-93f5cf205be6}"
    }
  }
}