Examine source code of Close Multiple tabs!

{
  "manifest_version": 2,
  "name": "Close Multiple tabs!",
  "version": "1.1resigned1",
  "permissions": [
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_title": "closeTab!"
  },
  "page_action": {
    "browser_style": true
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{d2e34faa-3fd8-4bc2-b156-62969d9fee11}"
    }
  }
}