This isn't a zXML error. You must pass a value to send() in Firefox, even if the value is null. So you should be doing:

oReq.send(null);

In firefox 2.0+, on Windows, you cannot do a GET XMLHttpRequest and send(), you must send(null). Been getting away with this for months, until tonight when Firefox auto-upgraded.

p2p.wrox.com Forums - zxml.js exception in Firefox but works fine in IE