To trigger this issue with the attached test case a11y must be enabled.

Reduced with m-c:
BuildID=20190327175114
SourceStamp=00c5ef76d951a834ccf87c83d04e0878f6c2c515

Is this a regression from bug 205202?

Assertion failure: list && !list->IsDirty(), at src/layout/generic/nsBulletFrame.cpp:822

#0 nsBulletFrame::Ordinal() const src/layout/generic/nsBulletFrame.cpp:822:3
#1 nsBulletFrame::GetSpokenText(nsTSubstring<char16_t>&) src/layout/generic/nsBulletFrame.cpp:1261:31
#2 nsBlockFrame::GetSpokenMarkerText(nsTSubstring<char16_t>&) const src/layout/generic/nsBlockFrame.cpp:6774:17
#3 mozilla::a11y::HTMLListBulletAccessible::AppendTextTo(nsTSubstring<char16_t>&, unsigned int, unsigned int) src/accessible/html/HTMLListAccessible.cpp:149:31
#4 mozilla::a11y::nsAccUtils::TextLength(mozilla::a11y::Accessible*) src/accessible/base/nsAccUtils.cpp:388:16
#5 mozilla::a11y::HyperTextAccessible::GetChildOffset(unsigned int, bool) const src/accessible/generic/HyperTextAccessible.cpp:1905:19
#6 mozilla::a11y::NotificationController::QueueMutationEvent(mozilla::a11y::AccTreeMutationEvent*) src/accessible/base/NotificationController.cpp:206:46
#7 mozilla::a11y::TreeMutation::BeforeRemoval(mozilla::a11y::Accessible*, bool) src/accessible/base/EventTree.cpp:86:21
#8 mozilla::a11y::DocAccessible::ContentRemoved(mozilla::a11y::Accessible*) src/accessible/generic/DocAccessible.cpp:1842:6
#9 mozilla::a11y::DocAccessible::ContentRemoved(nsIContent*) src/accessible/generic/DocAccessible.cpp:1871:5
#10 nsAccessibilityService::ContentRemoved(nsIPresShell*, nsIContent*) src/accessible/base/nsAccessibilityService.cpp:538:15
#11 nsCSSFrameConstructor::ContentRemoved(nsIContent*, nsIContent*, nsCSSFrameConstructor::RemoveFlags) src/layout/base/nsCSSFrameConstructor.cpp:7653:19
#12 mozilla::PresShell::ContentRemoved(nsIContent*, nsIContent*) src/layout/base/PresShell.cpp:4353:22
#13 nsNodeUtils::ContentRemoved(nsINode*, nsIContent*, nsIContent*) src/dom/base/nsNodeUtils.cpp:206:3
#14 nsINode::RemoveChildNode(nsIContent*, bool) src/dom/base/nsINode.cpp:1781:5
#15 nsINode::ReplaceOrInsertBefore(bool, nsINode*, nsINode*, mozilla::ErrorResult&) src/dom/base/nsINode.cpp:2115:18
#16 mozilla::dom::Node_Binding::appendChild(JSContext*, JS::Handle<JSObject*>, nsINode*, JSJitMethodCallArgs const&) src/obj-firefox/dom/bindings/NodeBinding.cpp:1021:45
#17 bool mozilla::dom::binding_detail::GenericMethod<mozilla::dom::binding_detail::NormalThisPolicy, mozilla::dom::binding_detail::ThrowExceptions>(JSContext*, unsigned int, JS::Value*) src/dom/bindings/BindingUtils.cpp:3144:13
#18 CallJSNative(JSContext*, bool (*)(JSContext*, unsigned int, JS::Value*), JS::CallArgs const&) src/js/src/vm/Interpreter.cpp:442:13
#19 js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) src/js/src/vm/Interpreter.cpp:534:12
#20 InternalCall(JSContext*, js::AnyInvokeArgs const&) src/js/src/vm/Interpreter.cpp:589:10
#21 Interpret(JSContext*, js::RunState&) src/js/src/vm/Interpreter.cpp:3075:16
#22 js::RunScript(JSContext*, js::RunState&) src/js/src/vm/Interpreter.cpp:422:10
#23 js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) src/js/src/vm/Interpreter.cpp:562:13
#24 InternalCall(JSContext*, js::AnyInvokeArgs const&) src/js/src/vm/Interpreter.cpp:589:10
#25 js::Call(JSContext*, JS::Handle<JS::Value>, JS::Handle<JS::Value>, js::AnyInvokeArgs const&, JS::MutableHandle<JS::Value>) src/js/src/vm/Interpreter.cpp:605:8
#26 JS::Call(JSContext*, JS::Handle<JS::Value>, JS::Handle<JS::Value>, JS::HandleValueArray const&, JS::MutableHandle<JS::Value>) src/js/src/jsapi.cpp:2621:10
#27 mozilla::dom::EventListener::HandleEvent(JSContext*, JS::Handle<JS::Value>, mozilla::dom::Event&, mozilla::ErrorResult&) src/obj-firefox/dom/bindings/EventListenerBinding.cpp:52:8
#28 void mozilla::dom::EventListener::HandleEvent<mozilla::dom::EventTarget*>(mozilla::dom::EventTarget* const&, mozilla::dom::Event&, mozilla::ErrorResult&, char const*, mozilla::dom::CallbackObject::ExceptionHandling, JS::Realm*) src/obj-firefox/dist/include/mozilla/dom/EventListenerBinding.h:66:12
#29 mozilla::EventListenerManager::HandleEventSubType(mozilla::EventListenerManager::Listener*, mozilla::dom::Event*, mozilla::dom::EventTarget*) src/dom/events/EventListenerManager.cpp:1038:43
#30 mozilla::EventListenerManager::HandleEventInternal(nsPresContext*, mozilla::WidgetEvent*, mozilla::dom::Event**, mozilla::dom::EventTarget*, nsEventStatus*, bool) src/dom/events/EventListenerManager.cpp:1239:17
#31 mozilla::EventTargetChainItem::HandleEvent(mozilla::EventChainPostVisitor&, mozilla::ELMCreationDetector&) src/dom/events/EventDispatcher.cpp:351:17
#32 mozilla::EventTargetChainItem::HandleEventTargetChain(nsTArray<mozilla::EventTargetChainItem>&, mozilla::EventChainPostVisitor&, mozilla::EventDispatchingCallback*, mozilla::ELMCreationDetector&) src/dom/events/EventDispatcher.cpp:553:16
#33 mozilla::EventDispatcher::Dispatch(nsISupports*, nsPresContext*, mozilla::WidgetEvent*, mozilla::dom::Event*, nsEventStatus*, mozilla::EventDispatchingCallback*, nsTArray<mozilla::dom::EventTarget*>*) src/dom/events/EventDispatcher.cpp:1048:11
#34 mozilla::EventDispatcher::DispatchDOMEvent(nsISupports*, mozilla::WidgetEvent*, mozilla::dom::Event*, nsPresContext*, nsEventStatus*) src/dom/events/EventDispatcher.cpp
#35 nsINode::DispatchEvent(mozilla::dom::Event&, mozilla::dom::CallerType, mozilla::ErrorResult&) src/dom/base/nsINode.cpp:1025:17
#36 nsContentUtils::DispatchEvent(mozilla::dom::Document*, nsISupports*, nsTSubstring<char16_t> const&, mozilla::CanBubble, mozilla::Cancelable, mozilla::Composed, mozilla::Trusted, bool*, mozilla::ChromeOnlyDispatch) src/dom/base/nsContentUtils.cpp:4063:28
#37 nsContentUtils::DispatchTrustedEvent(mozilla::dom::Document*, nsISupports*, nsTSubstring<char16_t> const&, mozilla::CanBubble, mozilla::Cancelable, mozilla::Composed, bool*) src/dom/base/nsContentUtils.cpp:4033:10
#38 mozilla::dom::Document::DispatchContentLoadedEvents() src/dom/base/Document.cpp:4708:3
#39 mozilla::detail::RunnableMethodImpl<mozilla::dom::Document*, void (mozilla::dom::Document::*)(), true, (mozilla::RunnableKind)0>::Run() src/obj-firefox/dist/include/nsThreadUtils.h:1174:13
#40 mozilla::SchedulerGroup::Runnable::Run() src/xpcom/threads/SchedulerGroup.cpp:295:32
#41 nsThread::ProcessNextEvent(bool, bool*) src/xpcom/threads/nsThread.cpp:1180:14
#42 NS_ProcessNextEvent(nsIThread*, bool) src/xpcom/threads/nsThreadUtils.cpp:482:10
#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:933:20
#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:771:34
#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:263:18

Yes it is. Though maybe it's an a11y issue.

To trigger this issue with the attached test case a11y must be enabled.

How do I enable a11y to trigger this assertion?

Is this a regression from bug 205202?

Yes, bug 205202 added this assertion intentionally for this reason -
to catch any callers from accessing an inconsistent counter state.

(In reply to Mats Palmgren (:mats) from comment #2)

How do I enable a11y to trigger this assertion?

We set GNOME_ACCESSIBILITY=1 during fuzzing runs to enable a11y

Let's be a little lenient with calls from a11y for now.
We can revisit this after nsBulletFrame is gone (bug 1542807).

Hmm, it seems every time I add a new crashtest the Android TV run times out:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=ebe070afb9522ffe064b41cce65b5888dcf7b17f&selectedJob=238840946
I'm betting that's a bug in the test framework/machines that I can safely ignore.

https://hg.mozilla.org/mozilla-central/rev/2c05c6e88f82