Firebase未在Chrome扩展程序内容脚本中加载

我正在尝试在Chrome扩展程序的内容脚本中加载Firebase:

var script   = document.createElement("script")
script.type  = "text/javascript"
script.src   = "https://cdn.firebase.com/js/client/1.1.2/firebase.js"
document.body.appendChild(script)

firebaseUrl = 'https://foobar.firebaseio.com'
root = new Firebase(firebaseUrl)

但我收到错误消息:

Uncaught ReferenceError: Firebase is not defined 

我究竟做错了什么？