debug XmlHttpRequest problem in Firefox extension

I am trying to add a feature to my Firefox extension, but have run into a problem. In most places in the extension, ajax (XmlHttpRequest) calls to my server work fine. With the new feature, the extension uses XmlHttpRequest to fetch some javascript code from the server. I get a "syntax error" some time between the launching of the XmlHttpRequest request and the invoking of the onload handler. Your job is to fix the problem. If you bid, please tell me about what experience you have working with Firefox extensions.