You have to remember that IE invented XMLHttp and Mozilla just duplicated its efforts. The fact that it's now going through a working draft at W3C doesn't mean that that is the way it *should* be.
My guess is that Firefox is using the old tried and true method of considering a null or undefined argument to be false (!!null or !!undefined).
Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/