This bug is for crash report bp-639e75e3-984e-48f1-8067-7c9260200308.

Top 10 frames of crashing thread:

0 xul.dll mozilla::UntrustedModulesProcessor::CompleteProcessing toolkit/xre/UntrustedModulesProcessor.cpp:883
1 xul.dll mozilla::MozPromise<mozilla::Maybe<mozilla::UntrustedModulesProcessor::ModulesMapResultWithLoads>, nsresult, 1>::ThenValue<`lambda at Z:/task_1583661969/build/src/toolkit/xre/UntrustedModulesProcessor.cpp:527:9', `lambda at Z:/task_1583661969/build/src/toolkit/xre/UntrustedModulesProcessor.cpp:536:9'>::DoResolveOrRejectInternal xpcom/threads/MozPromise.h:727
2 xul.dll mozilla::MozPromise<mozilla::Maybe<mozilla::UntrustedModulesProcessor::ModulesMapResultWithLoads>, nsresult, 1>::ThenValueBase::ResolveOrRejectRunnable::Run xpcom/threads/MozPromise.h:403
3 xul.dll nsThread::ProcessNextEvent xpcom/threads/nsThread.cpp:1220
4 xul.dll NS_ProcessNextEvent xpcom/threads/nsThreadUtils.cpp:481
5 xul.dll mozilla::ipc::MessagePumpForNonMainThreads::Run ipc/glue/MessagePump.cpp:332
6 xul.dll MessageLoop::RunHandler ipc/chromium/src/base/message_loop.cc:308
7 xul.dll MessageLoop::Run ipc/chromium/src/base/message_loop.cc:290
8 xul.dll static nsThread::ThreadFunc xpcom/threads/nsThread.cpp:464
9 nss3.dll _PR_NativeRunThread nsprpub/pr/src/threads/combined/pruthr.c:399

There are 3 crashes (from 2 installations) in nightly 75 with buildid 20200308095244. In analyzing the backtrace, the regression may have been introduced by patch [1] to fix bug 1605478.
The moz_crash_reason is always MOZ_DIAGNOSTIC_ASSERT(!event.IsTrusted()).

[1] https://hg.mozilla.org/mozilla-central/rev?node=996f931d61b6

There is one crash: https://crash-stats.mozilla.com/report/index/8465f7ee-b0da-41ae-9737-0f2ce0200308 where the crash_reason is MOZ_DIAGNOSTIC_ASSERT(false) (Something else)

These crashes were purposely introduced to narrow down the root cause of bug 1605478. So this is not a regression. Once we get necessary information, we back out the change. Let me resolve this as a dup of bug 1605478.

Actually having instances of MOZ_DIAGNOSTIC_ASSERT(!event.IsTrusted()) is really helpful. I think I got how this happened.