Below is the stack trace for the JVM hang on Vista when running with firefox.exe. The complete crash dump can be downloaded from http://javaweb.sfbay/~gmanwani/ms-cabs/jvm-busyhang/0298138434.cab

ntdll!KiFastSystemCallRet
ntdll!NtWaitForSingleObject+0xc
kernel32!WaitForSingleObjectEx+0xfa
kernel32!WaitForSingleObject+0x12
jvm!os::PlatformEvent::park+0x9f [c:\build_area\jdk6\hotspot\src\os\win32\vm\os_win32.cpp @ 3544]
jvm!ObjectMonitor::EnterI+0x136 [c:\build_area\jdk6\hotspot\src\share\vm\runtime\synchronizer.cpp @ 2527]
jvm!ObjectMonitor::enter+0x132 [c:\build_area\jdk6\hotspot\src\share\vm\runtime\synchronizer.cpp @ 2888]
jvm!ObjectSynchronizer::slow_enter+0x63 [c:\build_area\jdk6\hotspot\src\share\vm\runtime\synchronizer.cpp @ 980]
jvm!ObjectSynchronizer::fast_enter+0x4f [c:\build_area\jdk6\hotspot\src\share\vm\runtime\synchronizer.cpp @ 904]
jvm!InterpreterRuntime::monitorenter+0x57 [c:\build_area\jdk6\hotspot\src\share\vm\interpreter\interpreterruntime.cpp @ 519]
WARNING: Frame IP not in any known module. Following frames may be wrong.
0x5a1125f
0x5a02c71
0x5a02c71
jvm!ThreadStateTransition::transition+0x15 [c:\build_area\jdk6\hotspot\src\share\vm\runtime\interfacesupport.hpp @ 95]
jvm!JavaCallWrapper::JavaCallWrapper+0x7c [c:\build_area\jdk6\hotspot\src\share\vm\runtime\javacalls.cpp @ 42]
jvm!os::os_exception_wrapper+0x61 [c:\build_area\jdk6\hotspot\src\os_cpu\win32_i486\vm\os_win32_i486.cpp @ 53]
jvm!JavaCalls::call+0x1e [c:\build_area\jdk6\hotspot\src\share\vm\runtime\javacalls.cpp @ 284]
jvm!jni_invoke_nonstatic+0x1eb [c:\build_area\jdk6\hotspot\src\share\vm\prims\jni.cpp @ 1054]
jvm!jni_CallVoidMethodV+0xb3 [c:\build_area\jdk6\hotspot\src\share\vm\prims\jni.cpp @ 1342]
jpinscp!JNIEnv_::CallVoidMethod+0x1a [c:\build_area\jdk6\control\build\windows-i586\include\jni.h @ 1038]
jpinscp!JavaPluginObject::destroyPlugin+0x72 [c:\build_area\jdk6\deploy\src\plugin\win32\nscore\javapluginobject.cpp @ 329]
jpinscp!CJavaPlugin::Destroy+0x25 [c:\build_area\jdk6\deploy\src\plugin\win32\nscore\cjavaplugin.cpp @ 514]
jpioji!CNSAdapter_JavaPlugin::Destroy+0x1a [c:\build_area\jdk6\deploy\src\plugin\share\adapter\common\cnsadapter_javaplugin.cpp @ 206]
firefox+0x19e10e
firefox+0x19ebd1
firefox+0x19e00e
firefox+0x19e183
firefox+0x1be4ca
firefox+0x19e06b
firefox+0x1a9bb8
firefox+0x1aa231
firefox+0x1ff2dd
firefox+0x19c669
firefox+0x19c8a2
xpcom_core+0x2e24b
user32!InternalCallWinProc+0x23
user32!UserCallWinProcCheckWow+0x14b
user32!DispatchMessageWorker+0x322
user32!DispatchMessageW+0xf
firefox+0x119492
firefox+0x400d4a
firefox+0x1012
kernel32!BaseThreadInitThunk+0xe
ntdll!_RtlUserThreadStart+0x23