xmlhttprequest, firefox hangs on 3rd call

[simonB2007]

Looking for help with a scrip that works on IE, but suffers in Firefox.

URL : http://www.outofthebluesolutions.co.uk/tm/t.asp
(opens pop-up debugger)

Each run of script makes calls to "f_request" (consecutively)
Always calls to same path, but varying send strings.

Show Error:
Correct 1st run of code (auto on page load):
xmlhttprequest returns "Art" and adds to the tree.
Correct 2nd run (click '+' next to ART) :
xmlhttprequest re-used and returns 2 more nodes which are added to the tree.
ERROR 3rd run (click '+' next to CERAMIC) :
xmlhttprequest hangs when attempting open() call.

Most mysterious as the req object is .abort() before each call, and the calling path is hard-coded so no change.

Have tried several setRequestHeader things to prevent caching etc etc ... none to success (included in code below).

Reference Material Below...
(1) Javascript code
(2) .asp script generating responses
(3) Sample call strings (used in req.send(send)) and responses

Hope someone can help as i'm getting hopelessly lost !!
Many thanks
Simon
======================= t.asp
function f_request(s_inData, s_path){
//report bad call
if(!s_inData){debug("***false call");return false}
// if xmlhttprequest object exists, abort ready for re-use
// else create new request object
if(req){
debug("f_request : Aborting & Resusing");
req.abort();
}else{
debug("f_request : Creating New xmlHttpRequest Object");

try {req = new ActiveXObject("Msxml2.XMLHTTP");}
catch (e) {
try {req = new ActiveXObject("Microsoft.XMLHTTP");}
catch (e) {req = false;}
}
if (!req && typeof req !='undefined') {
try {req = new XMLHttpRequest();}
catch (e) {req=false;}
}
if (!req && window.createRequest) {
try {req = window.createRequest();}
catch (e) {req=false;}
}
}

fullPath = "treeIncludes/NDR.asp";
req.open("POST",fullPath,true);
req.setRequestHeader("Cache-Control", "no-cache");
//req.setRequestHeader("Connection", "close");
req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

req.onreadystatechange=sendDone;
req.send(s_inData);
}



======================= ndr.asp (data provider)

<%
Response.ExpiresAbsolute = #2000-01-01#
Response.AddHeader "pragma", "no-cache"
Response.AddHeader "cache-control", "private, no-cache, must-revalidate"
i_usr = request.form("usr")
s_cString = request.form("c")
s_filters= request.form("f")
///// o_cmd is user def function
Set o_cmd = new c_cmd
s_spName = "web_treeFeed_sp"
i_headers =0
o_cmd.CommandTextType s_spName , &H0004
o_cmd.addParamCrit "Usr", adInteger, adparaminput, 4, i_usr
o_cmd.addParamCrit "cString", adVarchar, adparaminput, 12, s_cString
o_cmd.addParamCrit "filters", advarchar, adparaminput, 12, s_filters
o_cmd.setHeaders(i_headers)
s_result = o_cmd.GetData()
Set o_cmd = Nothing
/////
Response.ContentType = "text/xml"
s_out = "<ReturnValue>" &_
s_result &_
"</ReturnValue>"
Response.write s_out
%>
=========================

call send strings ....
inData = usr=1&c=2,1,3,4,5&f=-1
inData = usr=1&c=2,1,3,4,5&f=-1,1
inData = usr=1&c=2,1,3,4,5&f=-1,1,1
scripted responses.....
<ReturnValue>0,1,Art#</ReturnValue>
<ReturnValue>0,1,Ceramic#0,2,Paint-Pencil#</ReturnValue>
<ReturnValue>0,4,Stoneware#</ReturnValue>