###@###.### indicates that with the 6u4 early access build, running an arbitrary applet on Linux (and, presumably, Solaris) locks up the web browser completely. The browser does not repaint and must be manually killed:
The same "firefox dies but the applet keeps running" symptom can be reproduced with the Chess game at java.com:
Click "TRY IT NOW", then click "Play as Guest". This will start the Applet. After a while (sometimes right away), the browser will stop responding to all input and repaint events and will have to be killed. The Applet continues to run. This happens on Linux, but not on Windows Vista. I didn't try any other OS.