Identical behavior here on same websites described, <http://websdr.ewi.utwente.nl:8901/>. <http://javaboutique.internet.com/Guitar/>both with FireFox and Windows Explorer browsers, most recent standard [not testing or beta] versions of both.
Java Plug-in 1.6.0_15 JRE version 1.6.0_15-b03 java script enabled Core 2 Duo Windows XP Professional SP3