89 + 90 affected, 90% of crashes on Windows 7, 40% in the first minute.

Crash report: https://crash-stats.mozilla.org/report/index/ebfe54af-8ab6-4375-bfe0-45f270210423

MOZ_CRASH Reason: MOZ_DIAGNOSTIC_ASSERT(false) (mScheduledDelayedRunnables.RemoveElement(aRunnable))

Top 10 frames of crashing thread:

0 xul.dll nsThread::OnDelayedRunnableRan xpcom/threads/nsThread.cpp:1404
1 xul.dll mozilla::DelayedRunnable::Notify xpcom/threads/DelayedRunnable.cpp:61
2 xul.dll nsTimerImpl::Fire xpcom/threads/nsTimerImpl.cpp:565
3 xul.dll nsTimerEvent::Run xpcom/threads/TimerThread.cpp:252
4 xul.dll mozilla::XPCOMThreadWrapper::Runner::Run xpcom/threads/AbstractThread.cpp:211
5 xul.dll mozilla::TaskController::DoExecuteNextTaskOnlyMainThreadInternal xpcom/threads/TaskController.cpp:757
6 xul.dll mozilla::detail::RunnableFunction<`lambda at /builds/worker/checkouts/gecko/xpcom/threads/TaskController.cpp:135:7'>::Run xpcom/threads/nsThreadUtils.h:534
7 xul.dll nsThread::ProcessNextEvent xpcom/threads/nsThread.cpp
8 xul.dll mozilla::ipc::MessagePump::Run ipc/glue/MessagePump.cpp:87
9 xul.dll MessageLoop::RunHandler ipc/chromium/src/base/message_loop.cc:328

I think this is the same assert as bug 1703978.