Crash report: https://crash-stats.mozilla.org/report/index/af459dbd-ed75-4cbf-bfaf-65d510200911

Top 10 frames of crashing thread:

0 libsystem_kernel.dylib __psynch_cvwait 
1 libsystem_pthread.dylib _pthread_cond_wait 
2 libsystem_pthread.dylib _pthread_ulock_cond_cleanup 
3 libmozglue.dylib mozilla::detail::ConditionVariableImpl::wait mozglue/misc/ConditionVariable_posix.cpp:108
4 XUL mozilla::TaskController::GetRunnableForMTTask xpcom/threads/TaskController.cpp:280
5 XUL nsThread::ProcessNextEvent xpcom/threads/nsThread.cpp:1156
6 libmozglue.dylib BaseAllocator::malloc memory/build/mozjemalloc.cpp:4055
7 libmozglue.dylib mozilla::detail::MutexImpl::unlock mozglue/misc/Mutex_posix.cpp:185
8 XUL NS_ProcessNextEvent xpcom/threads/nsThreadUtils.cpp:513
9 XUL mozilla::layers::CompositorThreadHolder::Shutdown gfx/layers/ipc/CompositorThread.cpp:129

• I don't know the exact component for this issue, feel free to change it.
• This happened while trying to reproduce bug 1580117 and had around 50-60 separate windows
• When I quit Firefox, it froze around 30 seconds then it crashed.
• Happened on macOS 11 and Firefox 81.0b9

Ran into this when trying to reproduce bug 1679116.

about 25% of crashes are Firefox, the rest Thunderbird

But are they the same issue?
bp-6a8d6c3c-3135-4f8a-b5a1-32f6b0210228 Thunderbird
bp-5ebbbc4e-7a0a-4920-a67a-35c990210228 Firefox

(In reply to Andrei Oprea [:andreio] from comment #1)

Ran into this when trying to reproduce bug 1679116.

Andrei, Can you still reproduce when using a newer version? What is your crash ID?

This crash signature is MacOS 10.16 only, so for Thunderbird, this crash signature might have the same cause as bug 1655236, whose original crash signature does not include MacOS 10.16

I can't trigger it anymore. Seems the behaviour of the modals changed recently and now you can't get the default browser prompt and the website authentication prompt to show up at the same time. My crash id 9749ce26-617a-4959-a71f-8dabe0201214

The crash rate for both signatures, and both Thunderbird and Firefox, has dropped significantly in the past week:

• https://crash-stats.mozilla.org/signature/?signature=shutdownhang%20%7C%20__psynch_cvwait%20%7C%20_pthread_cond_wait%20%7C%20_pthread_ulock_cond_cleanup&date=%3E%3D2021-05-21T01%3A21%3A00.000Z&date=%3C2021-06-21T01%3A21%3A00.000Z#graphs
• https://crash-stats.mozilla.org/signature/?signature=shutdownhang%20%7C%20__psynch_cvwait%20%7C%20_pthread_cond_wait%20%7C%20pthread_cond_signal_thread_np%20%7C%20%3Cname%20omitted%3E%20%7C%20mozilla%3A%3Adetail%3A%3AMutexImpl%3A%3Aunlock%20%7C%20XPCArrayHomogenizer%3A%3AGetTypeForArray&date=%3E%3D2021-05-21T01%3A21%3A00.000Z&date=%3C2021-06-21T01%3A21%3A00.000Z#graphs

Closing because no crashes reported for 12 weeks.