I figured out a bee in my bonnet. For awhile this error has been plaguing me:
Error: uncaught exception: [Exception... "Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIXMLHttpRequest.status]" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: http://bunny.jonnay.net/projects/TileEditor/project.js :: checkStatus :: line 47" data: no]
Turns out It was because I wasn't sending an empty string after my XmlHttpRequest open. That empty send is important.