User reports that the testvm applet (http://java.com/en/download/help/testvm.xml) has crashed when running Mustang B47 on Windows XP SP2 and Firefox 1.0.6. This problem cannot be reproduced on my machine.
http://forums.java.net/jive/thread.jspa?threadID=1184&tstart=0

Here is the Java Console printout:
Trace level set to 5: all ... completed.
basic: Stopping applet ...
basic: Removed progress listener: sun.plugin.util.GrayBoxPainter@c24c0
basic: Finding information ...
basic: Releasing classloader: sun.plugin.ClassLoaderInfo@108786b, refcount=0
basic: Caching classloader: sun.plugin.ClassLoaderInfo@108786b
basic: Current classloader cache size: 1
basic: Done ...
basic: Joining applet thread ...
basic: Destroying applet ...
basic: Disposing applet ...
basic: Quiting applet ...
basic: Joined applet thread ...
basic: Unregistered modality listener
Exception in thread "Loading System Properties" java.security.AccessControlException: access denied (java.lang.RuntimePermission modifyThread)
at java.security.AccessControlContext.checkPermission(Unknown Source)
at java.security.AccessController.checkPermission(Unknown Source)
at java.lang.SecurityManager.checkPermission(Unknown Source)
at sun.applet.AppletSecurity.checkAccess(Unknown Source)
at java.lang.Thread.checkAccess(Unknown Source)
at java.lang.Thread.setPriority(Unknown Source)
at testvm.run(testvm.java:262)
at java.lang.Thread.run(Unknown Source)
Exception in thread "Thread-23" java.security.AccessControlException: access denied (java.lang.RuntimePermission modifyThread)
at java.security.AccessControlContext.checkPermission(Unknown Source)
at java.security.AccessController.checkPermission(Unknown Source)
at java.lang.SecurityManager.checkPermission(Unknown Source)
at sun.applet.AppletSecurity.checkAccess(Unknown Source)
at java.lang.Thread.checkAccess(Unknown Source)
at java.lang.Thread.setPriority(Unknown Source)
at testvm.run(testvm.java:262)
at java.lang.Thread.run(Unknown Source)
basic: Registered modality listener
basic: Referencing classloader: sun.plugin.ClassLoaderInfo@108786b, refcount=1
basic: Added progress listener: sun.plugin.util.GrayBoxPainter@19b5217
basic: Loading applet ...
basic: Initializing applet ...
basic: Starting applet ...
TestVM 3.0 kc
Copyright (c) 2004 Sun Microsystems, Inc.
All Rights Reserved.
basic: Loaded image: http://java.com/en/img/download/t1.gif
basic: Loaded image: http://java.com/en/img/download/t2.gif
basic: Loaded image: http://java.com/en/img/download/t3.gif
basic: Loaded image: http://java.com/en/img/download/t4.gif
basic: Loaded image: http://java.com/en/img/download/t5.gif
basic: Loaded image: http://java.com/en/img/download/t6.gif
basic: Loaded image: http://java.com/en/img/download/t7.gif
liveconnect: JavaScript: UniversalBrowserRead enabled
liveconnect: JavaScript: UniversalJavaPermission enabled
liveconnect: JavaScript: UniversalBrowserRead enabled
liveconnect: JavaScript: UniversalJavaPermission enabled
basic: Stopping applet ...
basic: Removed progress listener: sun.plugin.util.GrayBoxPainter@19b5217
basic: Finding information ...
basic: Releasing classloader: sun.plugin.ClassLoaderInfo@108786b, refcount=0
basic: Caching classloader: sun.plugin.ClassLoaderInfo@108786b
basic: Current classloader cache size: 1
basic: Done ...
basic: Joining applet thread ...
basic: Destroying applet ...
basic: Disposing applet ...
basic: Quiting applet ...
basic: Joined applet thread ...
basic: Unregistered modality listener
basic: Registered modality listener
basic: Referencing classloader: sun.plugin.ClassLoaderInfo@108786b, refcount=1
basic: Added progress listener: sun.plugin.util.GrayBoxPainter@158f9d3
basic: Loading applet ...
basic: Initializing applet ...
basic: Starting applet ...
TestVM 3.0 kc
Copyright (c) 2004 Sun Microsystems, Inc.
All Rights Reserved.
basic: Loaded image: http://java.com/en/img/download/t1.gif
basic: Loaded image: http://java.com/en/img/download/t2.gif
basic: Loaded image: http://java.com/en/img/download/t3.gif
basic: Loaded image: http://java.com/en/img/download/t4.gif
basic: Loaded image: http://java.com/en/img/download/t5.gif
basic: Loaded image: http://java.com/en/img/download/t6.gif
basic: Loaded image: http://java.com/en/img/download/t7.gif
liveconnect: JavaScript: UniversalBrowserRead enabled
liveconnect: JavaScript: UniversalJavaPermission enabled
liveconnect: JavaScript: UniversalBrowserRead enabled
liveconnect: JavaScript: UniversalJavaPermission enabled
liveconnect: JavaScript: UniversalBrowserRead enabled
liveconnect: JavaScript: UniversalJavaPermission enabled