It may be that Firefox is not sending the HTTP request at all. From the error message it looks like firefox checks against a few rules to see if the HTTP Request is valid, and if it isn't, it gives the error you see...

You may want to try downloading the Firefox Extension called Firebug (if you haven't).

Firebug Dev site: http://www.getfirebug.com/

Once installed an enabled you will be able to view all the HTTP Requests made by firefox. You can filter to view only the "XHR" - XML HTTP Requests.

See if the XHR Request is being sent or not.

BTW: Did you try sending the Content-Length HTTP Request Header?

[HTML]xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");[/HTML]