AJAX :: Getting Html Tags Output Instead Of Executed Html Tags

i'm doing ajax website using PostWebRequest() function, when i call any .aspx page to the target html element the output is coming fine in IE but in FF(FireFox) is coming in html format(html tags) IE output : [URL] FF output : [URL]

javascript functions :
function PostWebRequest(postPage, HTMLTarget, parameter) {
displayElement = $get(HTMLTarget);
displayElement.innerHTML = "<div style='text-align:center;'><br /><br /><br /><br /><br /><img src='images/ajax-loader.gif' algin='center' /></div>";
var wRequest = new Sys.Net.WebRequest();
wRequest.set_url(postPage);
wRequest.set_httpVerb("POST");
var body = parameter;
wRequest.set_body(body);
wRequest.get_headers()["Content-Length"] = body.length;
wRequest.add_completed(onWebRequestCompleted);
wRequest.invoke();
}
function onWebRequestCompleted(executor, eventArgs) {
if (executor.get_responseAvailable()) {
if (document.all) {
displayElement.innerHTML = executor.get_responseData();
}
else {
displayElement.textContent = executor.get_responseData();
}
}
else {
if (executor.get_timeOut()) {
alert("Timed Out");
}
else {
if (executor.get_aborted())
alert("Aborted");
}
}
}