Reduced with m-c:
BuildID=20190819150944
SourceStamp=1868b438bd8307c67a4b3f569239410b843722e0

Test case requires the following prefs:

• dom.allow_scripts_to_close_windows=true
• dom.disable_open_during_load=false

Assertion failure: !aAccessor || !aAccessor->IsDiscarded(), at src/docshell/base/BrowsingContext.cpp:789

#0 mozilla::dom::BrowsingContext::LoadURI(mozilla::dom::BrowsingContext*, nsDocShellLoadState*) src/docshell/base/BrowsingContext.cpp:788:3
#1 nsWindowWatcher::OpenWindowInternal(mozIDOMWindowProxy*, char const*, char const*, char const*, bool, bool, bool, nsIArray*, bool, bool, bool, nsDocShellLoadState*, mozilla::dom::BrowsingContext**) src/toolkit/components/windowwatcher/nsWindowWatcher.cpp:1195:12
#2 nsWindowWatcher::OpenWindow2(mozIDOMWindowProxy*, char const*, char const*, char const*, bool, bool, bool, nsISupports*, bool, bool, bool, nsDocShellLoadState*, mozilla::dom::BrowsingContext**) src/toolkit/components/windowwatcher/nsWindowWatcher.cpp:377:10
#3 nsGlobalWindowOuter::OpenInternal(nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, bool, bool, bool, bool, bool, nsIArray*, nsISupports*, nsDocShellLoadState*, bool, mozilla::dom::BrowsingContext**) src/dom/base/nsGlobalWindowOuter.cpp:7304:21
#4 nsGlobalWindowOuter::OpenJS(nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, mozilla::dom::BrowsingContext**) src/dom/base/nsGlobalWindowOuter.cpp:5786:10
#5 nsGlobalWindowOuter::OpenOuter(nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, mozilla::ErrorResult&) src/dom/base/nsGlobalWindowOuter.cpp:5759:12
#6 nsGlobalWindowInner::Open(nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, nsTSubstring<char16_t> const&, mozilla::ErrorResult&) src/dom/base/nsGlobalWindowInner.cpp:3744:3
#7 mozilla::dom::Window_Binding::open(JSContext*, JS::Handle<JSObject*>, nsGlobalWindowInner*, JSJitMethodCallArgs const&) src/obj-firefox/dom/bindings/WindowBinding.cpp:2868:59
#8 bool mozilla::dom::binding_detail::GenericMethod<mozilla::dom::binding_detail::MaybeGlobalThisPolicy, mozilla::dom::binding_detail::ThrowExceptions>(JSContext*, unsigned int, JS::Value*) src/dom/bindings/BindingUtils.cpp:3163:13
#9 CallJSNative(JSContext*, bool (*)(JSContext*, unsigned int, JS::Value*), JS::CallArgs const&) src/js/src/vm/Interpreter.cpp:447:13
#10 js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) src/js/src/vm/Interpreter.cpp:539:12
#11 InternalCall(JSContext*, js::AnyInvokeArgs const&) src/js/src/vm/Interpreter.cpp:594:10
#12 Interpret(JSContext*, js::RunState&) src/js/src/vm/Interpreter.cpp:3084:16
#13 js::RunScript(JSContext*, js::RunState&) src/js/src/vm/Interpreter.cpp:424:10
#14 js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) src/js/src/vm/Interpreter.cpp:567:13
#15 InternalCall(JSContext*, js::AnyInvokeArgs const&) src/js/src/vm/Interpreter.cpp:594:10
#16 js::Call(JSContext*, JS::Handle<JS::Value>, JS::Handle<JS::Value>, js::AnyInvokeArgs const&, JS::MutableHandle<JS::Value>) src/js/src/vm/Interpreter.cpp:610:8
#17 JS::Call(JSContext*, JS::Handle<JS::Value>, JS::Handle<JS::Value>, JS::HandleValueArray const&, JS::MutableHandle<JS::Value>) src/js/src/jsapi.cpp:2722:10
#18 mozilla::dom::EventHandlerNonNull::Call(JSContext*, JS::Handle<JS::Value>, mozilla::dom::Event&, JS::MutableHandle<JS::Value>, mozilla::ErrorResult&) src/obj-firefox/dom/bindings/EventHandlerBinding.cpp:267:37
#19 void mozilla::dom::EventHandlerNonNull::Call<nsCOMPtr<mozilla::dom::EventTarget> >(nsCOMPtr<mozilla::dom::EventTarget> const&, mozilla::dom::Event&, JS::MutableHandle<JS::Value>, mozilla::ErrorResult&, char const*, mozilla::dom::CallbackObject::ExceptionHandling, JS::Realm*) src/obj-firefox/dist/include/mozilla/dom/EventHandlerBinding.h:363:12
#20 mozilla::JSEventHandler::HandleEvent(mozilla::dom::Event*) src/dom/events/JSEventHandler.cpp:205:12
#21 mozilla::EventListenerManager::HandleEventSubType(mozilla::EventListenerManager::Listener*, mozilla::dom::Event*, mozilla::dom::EventTarget*) src/dom/events/EventListenerManager.cpp:1039:22
#22 mozilla::EventListenerManager::HandleEventInternal(nsPresContext*, mozilla::WidgetEvent*, mozilla::dom::Event**, mozilla::dom::EventTarget*, nsEventStatus*, bool) src/dom/events/EventListenerManager.cpp:1231:17
#23 mozilla::EventTargetChainItem::HandleEvent(mozilla::EventChainPostVisitor&, mozilla::ELMCreationDetector&) src/dom/events/EventDispatcher.cpp:349:17
#24 mozilla::EventTargetChainItem::HandleEventTargetChain(nsTArray<mozilla::EventTargetChainItem>&, mozilla::EventChainPostVisitor&, mozilla::EventDispatchingCallback*, mozilla::ELMCreationDetector&) src/dom/events/EventDispatcher.cpp:551:16
#25 mozilla::EventDispatcher::Dispatch(nsISupports*, nsPresContext*, mozilla::WidgetEvent*, mozilla::dom::Event*, nsEventStatus*, mozilla::EventDispatchingCallback*, nsTArray<mozilla::dom::EventTarget*>*) src/dom/events/EventDispatcher.cpp:1047:11
#26 FocusBlurEvent::Run() src/dom/base/nsFocusManager.cpp:1981:12
#27 nsContentUtils::AddScriptRunner(already_AddRefed<nsIRunnable>) src/dom/base/nsContentUtils.cpp:5246:13
#28 nsContentUtils::AddScriptRunner(nsIRunnable*) src/dom/base/nsContentUtils.cpp:5252:3
#29 nsFocusManager::FireFocusOrBlurEvent(mozilla::EventMessage, mozilla::PresShell*, nsISupports*, bool, bool, mozilla::dom::EventTarget*) src/dom/base/nsFocusManager.cpp:2129:5
#30 nsFocusManager::SendFocusOrBlurEvent(mozilla::EventMessage, mozilla::PresShell*, mozilla::dom::Document*, nsISupports*, unsigned int, bool, bool, mozilla::dom::EventTarget*) src/dom/base/nsFocusManager.cpp:2097:3
#31 nsFocusManager::Blur(nsPIDOMWindowOuter*, nsPIDOMWindowOuter*, bool, bool, nsIContent*) src/dom/base/nsFocusManager.cpp:1724:7
#32 nsFocusManager::WindowLowered(mozIDOMWindowProxy*) src/dom/base/nsFocusManager.cpp:761:23
#33 nsWebBrowser::FocusDeactivate() src/toolkit/components/browser/nsWebBrowser.cpp:1355:9
#34 mozilla::dom::BrowserChild::RecvDeactivate() src/dom/ipc/BrowserChild.cpp:1450:16
#35 mozilla::dom::PContentChild::OnMessageReceived(IPC::Message const&) src/obj-firefox/ipc/ipdl/PContentChild.cpp:11276:56
#36 mozilla::dom::ContentChild::OnMessageReceived(IPC::Message const&) src/dom/ipc/ContentChild.cpp:3811:25
#37 mozilla::ipc::MessageChannel::DispatchAsyncMessage(mozilla::ipc::ActorLifecycleProxy*, IPC::Message const&) src/ipc/glue/MessageChannel.cpp:2184:25
#38 mozilla::ipc::MessageChannel::DispatchMessage(IPC::Message&&) src/ipc/glue/MessageChannel.cpp:2108:9
#39 mozilla::ipc::MessageChannel::RunMessage(mozilla::ipc::MessageChannel::MessageTask&) src/ipc/glue/MessageChannel.cpp:1955:3
#40 mozilla::ipc::MessageChannel::MessageTask::Run() src/ipc/glue/MessageChannel.cpp:1986:13
#41 nsThread::ProcessNextEvent(bool, bool*) src/xpcom/threads/nsThread.cpp:1225:14
#42 NS_ProcessNextEvent(nsIThread*, bool) src/xpcom/threads/nsThreadUtils.cpp:486:10
#43 mozilla::ipc::MessagePump::Run(base::MessagePump::Delegate*) src/ipc/glue/MessagePump.cpp:88:21
#44 MessageLoop::RunInternal() src/ipc/chromium/src/base/message_loop.cc:315:10
#45 MessageLoop::Run() src/ipc/chromium/src/base/message_loop.cc:290:3
#46 nsBaseAppShell::Run() src/widget/nsBaseAppShell.cpp:137:27
#47 XRE_RunAppShell() src/toolkit/xre/nsEmbedFunctions.cpp:934:20
#48 mozilla::ipc::MessagePumpForChildProcess::Run(base::MessagePump::Delegate*) src/ipc/glue/MessagePump.cpp:238:9
#49 MessageLoop::RunInternal() src/ipc/chromium/src/base/message_loop.cc:315:10
#50 MessageLoop::Run() src/ipc/chromium/src/base/message_loop.cc:290:3
#51 XRE_InitChildProcess(int, char**, XREChildData const*) src/toolkit/xre/nsEmbedFunctions.cpp:769:34
#52 content_process_main(mozilla::Bootstrap*, int, char**) src/browser/app/../../ipc/contentproc/plugin-container.cpp:56:28
#53 main src/browser/app/nsBrowserApp.cpp:267:18

Tentatively moving all bugs whose summaries mention "Fission" (or other Fission-related keywords) but are not assigned to a Fission Milestone to the "?" triage milestone.

This will generate a lot of bugmail, so you can filter your bugmail for the following UUID and delete them en masse:

0ee3c76a-bc79-4eb2-8d12-05dc0b68e732

Another one related to discarded BC.

I wasn't able to reproduce this, and it looks like a dupe of bug 1574593.