This document describes some of the changes that have been made to Gecko's XMLHttpRequest implementation since version 1.7 (i.e., the version of Gecko that shipped with Firefox 1.0). These changes only apply to XUL extensions and XUL applications. They do not apply to web applications.
Changes to XMLHttpRequest.send
If you pass a nsIInputStream instance to the send method, then it must no longer include Content-Length and Content-Type headers at the beginning of the stream. Instead, the Content-Length header will be inferred from the length of the stream, and the Content-Type header must be specified manually by calling the setRequestHeader method. These requirements are more fully described here.
Changes to XMLHttpRequest.onreadystatechange
This attribute now has a type of nsIOnReadyStateChangeHandler instead of nsIOnReadystatechangehandler.