Note for bugmon GNOME_ACCESSIBILITY=1

Assertion failure: ipcDoc, at src/accessible/generic/Accessible.cpp:790

#0 0x7f0c060bd28f in mozilla::a11y::Accessible::HandleAccEvent(mozilla::a11y::AccEvent*) src/accessible/generic/Accessible.cpp:790:5
#1 0x7f0c0606c698 in mozilla::a11y::AccessibleWrap::HandleAccEvent(mozilla::a11y::AccEvent*) src/accessible/atk/AccessibleWrap.cpp:1049:29
#2 0x7f0c06096687 in nsEventShell::FireEvent(mozilla::a11y::AccEvent*) src/accessible/base/nsEventShell.cpp:43:15
#3 0x7f0c060dacaf in mozilla::a11y::RootAccessible::ProcessDOMEvent(mozilla::dom::Event*, nsINode*) src/accessible/generic/RootAccessible.cpp:326:7
#4 0x7f0c060908e2 in mozilla::a11y::NotificationController::WillRefresh(mozilla::TimeStamp) src/accessible/base/NotificationController.cpp:837:25
#5 0x7f0c04f6bace in nsRefreshDriver::Tick(mozilla::layers::BaseTransactionId<mozilla::VsyncIdType>, mozilla::TimeStamp) src/layout/base/nsRefreshDriver.cpp:2139:12
#6 0x7f0c04f73dd1 in TickDriver src/layout/base/nsRefreshDriver.cpp:357:13
#7 0x7f0c04f73dd1 in mozilla::RefreshDriverTimer::TickRefreshDrivers(mozilla::layers::BaseTransactionId<mozilla::VsyncIdType>, mozilla::TimeStamp, nsTArray<RefPtr<nsRefreshDriver> >&) src/layout/base/nsRefreshDriver.cpp:336:7
#8 0x7f0c04f73caf in mozilla::RefreshDriverTimer::Tick(mozilla::layers::BaseTransactionId<mozilla::VsyncIdType>, mozilla::TimeStamp) src/layout/base/nsRefreshDriver.cpp:351:5
#9 0x7f0c04f73258 in RunRefreshDrivers src/layout/base/nsRefreshDriver.cpp:799:5
#10 0x7f0c04f73258 in mozilla::VsyncRefreshDriverTimer::RefreshDriverVsyncObserver::TickRefreshDriver(mozilla::layers::BaseTransactionId<mozilla::VsyncIdType>, mozilla::TimeStamp) src/layout/base/nsRefreshDriver.cpp:722:16
#11 0x7f0c04f72b70 in mozilla::VsyncRefreshDriverTimer::RefreshDriverVsyncObserver::NotifyParentProcessVsync() src/layout/base/nsRefreshDriver.cpp:624:7
#12 0x7f0c04f725e9 in mozilla::VsyncRefreshDriverTimer::RefreshDriverVsyncObserver::NotifyVsync(mozilla::VsyncEvent const&) src/layout/base/nsRefreshDriver.cpp:545:9
#13 0x7f0c047616f6 in mozilla::dom::VsyncChild::RecvNotify(mozilla::VsyncEvent const&, float const&) src/dom/ipc/VsyncChild.cpp:68:15
#14 0x7f0c014a8210 in mozilla::dom::PVsyncChild::OnMessageReceived(IPC::Message const&) /builds/worker/workspace/obj-build/ipc/ipdl/PVsyncChild.cpp:178:54
#15 0x7f0c0124abac in mozilla::ipc::PBackgroundChild::OnMessageReceived(IPC::Message const&) /builds/worker/workspace/obj-build/ipc/ipdl/PBackgroundChild.cpp:6286:32
#16 0x7f0c00f00a3e in mozilla::ipc::MessageChannel::DispatchAsyncMessage(mozilla::ipc::ActorLifecycleProxy*, IPC::Message const&) src/ipc/glue/MessageChannel.cpp:2153:25
#17 0x7f0c00efcfbd in mozilla::ipc::MessageChannel::DispatchMessage(IPC::Message&&) src/ipc/glue/MessageChannel.cpp:2077:9
#18 0x7f0c00efe466 in mozilla::ipc::MessageChannel::RunMessage(mozilla::ipc::MessageChannel::MessageTask&) src/ipc/glue/MessageChannel.cpp:1925:3
#19 0x7f0c00eff1ab in mozilla::ipc::MessageChannel::MessageTask::Run() src/ipc/glue/MessageChannel.cpp:1956:13
#20 0x7f0c005cc57f in mozilla::RunnableTask::Run() src/xpcom/threads/TaskController.cpp:472:16
#21 0x7f0c005caaf3 in mozilla::TaskController::DoExecuteNextTaskOnlyMainThreadInternal(mozilla::detail::BaseAutoLock<mozilla::Mutex&> const&) src/xpcom/threads/TaskController.cpp:753:26
#22 0x7f0c005c9954 in mozilla::TaskController::ExecuteNextTaskOnlyMainThreadInternal(mozilla::detail::BaseAutoLock<mozilla::Mutex&> const&) src/xpcom/threads/TaskController.cpp:611:15
#23 0x7f0c005c9b07 in mozilla::TaskController::ProcessPendingMTTask(bool) src/xpcom/threads/TaskController.cpp:395:36
#24 0x7f0c005d00a6 in operator() src/xpcom/threads/TaskController.cpp:133:37
#25 0x7f0c005d00a6 in mozilla::detail::RunnableFunction<mozilla::TaskController::InitializeInternal()::$_3>::Run() /builds/worker/workspace/obj-build/dist/include/nsThreadUtils.h:534:5
#26 0x7f0c005e1657 in nsThread::ProcessNextEvent(bool, bool*) src/xpcom/threads/nsThread.cpp:1171:16
#27 0x7f0c005e7a9a in NS_ProcessNextEvent(nsIThread*, bool) src/xpcom/threads/nsThreadUtils.cpp:548:10
#28 0x7f0c00f06326 in mozilla::ipc::MessagePump::Run(base::MessagePump::Delegate*) src/ipc/glue/MessagePump.cpp:87:21
#29 0x7f0c00e71c63 in MessageLoop::RunInternal() src/ipc/chromium/src/base/message_loop.cc:335:10
#30 0x7f0c00e71b7d in RunHandler src/ipc/chromium/src/base/message_loop.cc:328:3
#31 0x7f0c00e71b7d in MessageLoop::Run() src/ipc/chromium/src/base/message_loop.cc:310:3
#32 0x7f0c04cbf7e8 in nsBaseAppShell::Run() src/widget/nsBaseAppShell.cpp:137:27
#33 0x7f0c065046d3 in XRE_RunAppShell() src/toolkit/xre/nsEmbedFunctions.cpp:902:20
#34 0x7f0c00f0720c in mozilla::ipc::MessagePumpForChildProcess::Run(base::MessagePump::Delegate*) src/ipc/glue/MessagePump.cpp:237:9
#35 0x7f0c00e71c63 in MessageLoop::RunInternal() src/ipc/chromium/src/base/message_loop.cc:335:10
#36 0x7f0c00e71b7d in RunHandler src/ipc/chromium/src/base/message_loop.cc:328:3
#37 0x7f0c00e71b7d in MessageLoop::Run() src/ipc/chromium/src/base/message_loop.cc:310:3
#38 0x7f0c065042a8 in XRE_InitChildProcess(int, char**, XREChildData const*) src/toolkit/xre/nsEmbedFunctions.cpp:733:34
#39 0x561c996b7f76 in content_process_main src/browser/app/../../ipc/contentproc/plugin-container.cpp:57:28
#40 0x561c996b7f76 in main src/browser/app/nsBrowserApp.cpp:306:18
#41 0x7f0c1540b0b2 in __libc_start_main /build/glibc-ZN95T4/glibc-2.31/csu/../csu/libc-start.c:308:16
#42 0x561c99695d1c in _start (/home/worker/builds/m-c-20210131211743-fuzzing-debug/firefox-bin+0x14d1c)

A Pernosco session is available here: https://pernos.co/debug/GS6uk5s2LflxiWyPZZohYQ/index.html

Bugmon Analysis:
Verified bug as reproducible on mozilla-central 20210304040740-eee3ec3004e4.
Failed to bisect testcase (Testcase reproduces on start build!):

Start: 1e4b37c9e558728666cf1d006d95677acc7f8153 (20200305041649)
End: c45b1e6bcd016bd6159170ad3d61523452c19218 (20210303041649)
BuildFlags: BuildFlags(asan=False, tsan=False, debug=True, fuzzing=False, coverage=False, valgrind=False)