I think I figured it out. I think that the issue crops up if your server sends a response header of ‘Content-Type’ set to ‘text/xml’ and you send no content. The XMLHTTPRequest object then says ‘oh it’s xml? fine – i’ll just PARSE THIS and store it in the reponseXML property.’ At that point, the XML Parser probably bombs and nothing catches it. But anyway that’s just a hunch and I’m only blogging about it because I’m bored. I’ll have test results available later.
Solved if content-type of the response is “text/plain” and content is say a space (” “) – empty string doesn’t work.
I am having the same issue, except that I’m not returning any entity body, so I have no content-type header. But http://www.w3.org/TR/XMLHttpRequest states that it should then default to be treated as xml. It also specifies that if the entity body is null, the responseXML should also be null. Looks like firefox isn’t following that.