Examine source code of Show Close Tab Button

{
  "manifest_version": 2,
  "name": "Show Close Tab Button",
  "version": "1.9.2",
  "description": "Show a close button on each active tab to close it. And you can add buttons and set the arrangement and position of buttons yourself.",
  "permissions": [
    "storage",
    "activeTab",
    "scripting",
    "tabs",
    "http://*/*",
    "https://*/*"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon-16.png",
      "48": "icons/icon-48.png",
      "128": "icons/icon-128.png"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "icons/icon-16.png",
    "icons/icon-48.png",
    "icons/icon-128.png"
  ],
  "icons": {
    "16": "icons/icon-16.png",
    "48": "icons/icon-48.png",
    "128": "icons/icon-128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "showclosetabbutton@github.io"
    }
  }
}