XmlHTTPRequest was created by Microsoft for OWA around 2000. I remember using JSRS (http://www.ashleyit.com/rs/) at the time until Mozilla started supporting XHR. Fine times :)
Very cool. From a quick test it looks like JSRS uses a hidden IFrame; in the case of POST it's the target for a scripted hidden form submission. The response is embedded in HTML though (as a textarea no less!), which doesn't make much sense to me.