Examine source code of Close Active Tab button

{
  "description": "Adds browser action icon to toolbar to close current tab.",
  "manifest_version": 2,
  "name": "close-active-tab",
  "version": "1.0",
  "homepage_url": "https://www.firefox.org",
  "icons": {
    "16": "icons/close-16.png",
    "32": "icons/close-32.png",
    "48": "icons/close-48.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_icon": "icons/close-32.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{d898ace8-2b0a-487a-b092-76f97bd232c4}"
    }
  }
}