UPDATE: This seemes to be a bug in FireFox 3.0.1
Works in IE8 beta 2 (currently my IE version).
Another argument for abstracting away JavaScript and let Gaia Ajax do your advanced Ajax. Umbraco could really need it sometimes. We are dual licensed, so please contact me, mr Hartvig, regarding a demo of Gaia Ajax. :-)
Want easy Ajax in Umbraco?Download and test with Gaia Ajax -
http://gaiaware.netOpen Source