When served as mime-type text/html, browsers make element.tagName uppercase, but when served as application/xhtml+xml at least firefox and opera make element.tagName lowercase per the spec.
The bug is at http://trac.mochikit.com/browser/mochikit/trunk/MochiKit/Style.js#L326 where it assumes "BODY" and "HTML" are uppercase. This causes firefox and opera to return NaN for the element position. Changing it to parent.tagName.toUpperCase() makes it work for me.
A quick grep of revision-1101 suggests that this bug might also exist in a few other places, but I haven't encountered it anywhere else yet.
thanks,
Wes Augur
wes@oxona.com