Bad XMLHttpRequest in Firefox (Nightly)

Tweeted a link to http://requirebin.com/?gist=5963363 and got a report that it didn't work. I checked it out in Firefox and saw this:

Error: Bad xmlhttprequest.XMLHttpRequest @ http://requirebin.com/bundle.js:21056

Which was thrown by a part of module browser-request:

var xmlhttprequest = require('./xmlhttprequest')
if(!xmlhttprequest || typeof xmlhttprequest !== 'object')
  throw new Error('Could not find ./xmlhttprequest')

var XHR = xmlhttprequest.XMLHttpRequest
if(!XHR)
  throw new Error('Bad xmlhttprequest.XMLHttpRequest')
if(! ('_object' in (new XHR)))
  throw new Error('This is not portable XMLHttpRequest')

Start by reproducing the Firefox Nightly failure from the linked requirebin example, then inspect bundle.js at line 21056 and the browser-request XMLHttpRequest checks shown in the issue. Determine why Firefox fails the portable XMLHttpRequest validation; done means the example loads successfully there without the reported error.