I was receiving the following error with only Firefox: XML Parsing Error: no element found. Basically, I was trying to visit a page that has no content, with Ajax.Request. The requested page performs a passive operation on the server, using just query string parameters, and it is not meant to return content. Since my doc type is xhtml, Firefox thinks that the document is an xml document and throws an error because it contains no data. So, as a quick fix, I just added this as the content of the page.
That fixes things, and since I don’t expect a return value, it’s ignored by my app. Hope it helps someone tracking down the same issue.