From http://www.w3schools.com (Copyright Refsnes Data)

E4X Browsers

The browser support for E4X is limited.

Limited Browser Support for E4X

None of the mainstream browsers are currently supporting E4X.

A beta version of the Mozilla engine (1.8) has limited support for E4X. The first browser to support E4X is expected to be Firefox 1.1.

Support for E4X is also expected in a later version of Internet Explorer.

In the next version of Firefox, Firefox 1.1 is expected to have relatively good support for E4X.

In the next chapter there is a list of examples, describing some of the things you can do with E4x in Firefox 1.1.

So far there is no indication for of E4X support in Internet Explorer 7.

It looks like Microsoft is going to support its own alternative to AJAX.

AJAX is a new web development technique for creating interactive web applications using JavaScript, XML, and CSS.

You can read more about AJAX in our AJAX tutorial.