Found with m-c: BuildID=20180510164324 SourceStamp=17db33b6a124422d43a9f518bea1bc62a698126b Assertion failure: XRE_IsParentProcess(), at src/xpcom/io/nsAnonymousTemporaryFile.cpp:87 0|libxul.so|NS_OpenAnonymousTemporaryNsIFile|hg:hg.mozilla.org/mozilla-central:xpcom/io/nsAnonymousTemporaryFile.cpp:87 1|libxul.so|NS_OpenAnonymousTemporaryFile|hg:hg.mozilla.org/mozilla-central:xpcom/io/nsAnonymousTemporaryFile.cpp:127 2|libxul.so|DataStruct::WriteCache|hg:hg.mozilla.org/mozilla-central:widget/nsTransferable.cpp:134 3|libxul.so|DataStruct::SetData|hg:hg.mozilla.org/mozilla-central:widget/nsTransferable.cpp:80 4|libxul.so|nsTransferable::SetTransferData|hg:hg.mozilla.org/mozilla-central:widget/nsTransferable.cpp:383 5|libxul.so|AppendString|hg:hg.mozilla.org/mozilla-central:dom/base/nsCopySupport.cpp:481 6|libxul.so|SelectionCopyHelper|hg:hg.mozilla.org/mozilla-central:dom/base/nsCopySupport.cpp:224 7|libxul.so|mozilla::dom::Selection::NotifySelectionListeners|hg:hg.mozilla.org/mozilla-central:dom/base/Selection.cpp:3469 8|libxul.so|nsFrameSelection::NotifySelectionListeners|hg:hg.mozilla.org/mozilla-central:layout/generic/nsFrameSelection.cpp:1998 9|libxul.so|mozilla::dom::Selection::EndBatchChanges|hg:hg.mozilla.org/mozilla-central:dom/base/Selection.cpp:3488 10|libxul.so|mozilla::dom::SelectionBatcher::~SelectionBatcher|hg:hg.mozilla.org/mozilla-central:dom/base/Selection.h:710 11|libxul.so|mozilla::dom::Selection::SelectAllChildren|hg:hg.mozilla.org/mozilla-central:dom/base/Selection.cpp:2951 12|libxul.so|mozilla::dom::Selection::SelectAllChildrenJS|hg:hg.mozilla.org/mozilla-central:dom/base/Selection.cpp:2928 13|libxul.so|mozilla::dom::SelectionBinding::selectAllChildren|s3:gecko-generated-sources:9efd41fd7c77c4767c73c88686aca6bab718785a6e360c4f20ce9446ae5e717dc980d0c1646c6e613a77e9c0755473e8cea3b2ff5c977494de2042104ea3b12a/dom/bindings/SelectionBinding.cpp:|792 14|libxul.so|mozilla::dom::binding_detail::GenericMethod<mozilla::dom::binding_detail::NormalThisPolicy, mozilla::dom::binding_detail::ThrowExceptions>|hg:hg.mozilla.org/mozilla-central:dom/bindings/BindingUtils.cpp:3260 15|libxul.so|js::CallJSNative|hg:hg.mozilla.org/mozilla-central:js/src/vm/JSContext-inl.h:280 16|libxul.so|js::InternalCallOrConstruct|hg:hg.mozilla.org/mozilla-central:js/src/vm/Interpreter.cpp:467 17|libxul.so|InternalCall|hg:hg.mozilla.org/mozilla-central:js/src/vm/Interpreter.cpp:516 18|libxul.so|js::jit::DoCallFallback|hg:hg.mozilla.org/mozilla-central:js/src/jit/BaselineIC.cpp:2382 19|||| 20|||| 21|||| 22|libxul.so|js::jit::EnterBaselineAtBranch|hg:hg.mozilla.org/mozilla-central:js/src/jit/BaselineJIT.cpp:151 23|libxul.so|Interpret|hg:hg.mozilla.org/mozilla-central:js/src/vm/Interpreter.cpp:2037 24|libxul.so|js::RunScript|hg:hg.mozilla.org/mozilla-central:js/src/vm/Interpreter.cpp:417 25|libxul.so|js::InternalCallOrConstruct|hg:hg.mozilla.org/mozilla-central:js/src/vm/Interpreter.cpp:489 26|libxul.so|InternalCall|hg:hg.mozilla.org/mozilla-central:js/src/vm/Interpreter.cpp:516 27|libxul.so|js::Call|hg:hg.mozilla.org/mozilla-central:js/src/vm/Interpreter.cpp:535 28|libxul.so|JS::Call|hg:hg.mozilla.org/mozilla-central:js/src/jsapi.cpp:2989 29|libxul.so|mozilla::dom::IdleRequestCallback::Call|s3:gecko-generated-sources:6d8b030a51faeab6c42cdfb86b6a63510df1deeaa3bd4969baee26e634bcbbf8cc839613cea75404b8500cc0d3fb88f63b73a01c1887f8b9ad3ad72f7ad70eca/dom/bindings/WindowBinding.cpp:|863 30|libxul.so|mozilla::dom::IdleRequestCallback::Call|s3:gecko-generated-sources:9f074db754301c9b1d9794d924eb1f72f85debc94e12d58a6105636967d0702aedb820e4bd9a8b8ef7b8e159ffe51086304a70a4c31fe08eba54ce25e37b2496/dist/include/mozilla/dom/WindowBinding.h:|720 31|libxul.so|mozilla::dom::IdleRequest::IdleRun|hg:hg.mozilla.org/mozilla-central:dom/base/IdleRequest.cpp:74 32|libxul.so|nsGlobalWindowInner::RunIdleRequest|hg:hg.mozilla.org/mozilla-central:dom/base/nsGlobalWindowInner.cpp:700 33|libxul.so|nsGlobalWindowInner::ExecuteIdleRequest|hg:hg.mozilla.org/mozilla-central:dom/base/nsGlobalWindowInner.cpp:730 34|libxul.so|nsThread::ProcessNextEvent|hg:hg.mozilla.org/mozilla-central:xpcom/threads/nsThread.cpp:1090 35|libxul.so|NS_ProcessNextEvent|hg:hg.mozilla.org/mozilla-central:xpcom/threads/nsThreadUtils.cpp:519 36|libxul.so|mozilla::ipc::MessagePump::Run|hg:hg.mozilla.org/mozilla-central:ipc/glue/MessagePump.cpp:97 37|libxul.so|MessageLoop::RunInternal|hg:hg.mozilla.org/mozilla-central:ipc/chromium/src/base/message_loop.cc:326 38|libxul.so|MessageLoop::Run|hg:hg.mozilla.org/mozilla-central:ipc/chromium/src/base/message_loop.cc:319 39|libxul.so|nsBaseAppShell::Run|hg:hg.mozilla.org/mozilla-central:widget/nsBaseAppShell.cpp:157 40|libxul.so|XRE_RunAppShell|hg:hg.mozilla.org/mozilla-central:toolkit/xre/nsEmbedFunctions.cpp:893 41|libxul.so|mozilla::ipc::MessagePumpForChildProcess::Run|hg:hg.mozilla.org/mozilla-central:ipc/glue/MessagePump.cpp:269 42|libxul.so|MessageLoop::RunInternal|hg:hg.mozilla.org/mozilla-central:ipc/chromium/src/base/message_loop.cc:326 43|libxul.so|MessageLoop::Run|hg:hg.mozilla.org/mozilla-central:ipc/chromium/src/base/message_loop.cc:319 44|libxul.so|XRE_InitChildProcess|hg:hg.mozilla.org/mozilla-central:toolkit/xre/nsEmbedFunctions.cpp:719 45|firefox|content_process_main|hg:hg.mozilla.org/mozilla-central:ipc/contentproc/plugin-container.cpp:50 46|firefox|main|hg:hg.mozilla.org/mozilla-central:browser/app/nsBrowserApp.cpp:282 47|libc-2.23.so||| 48|firefox|MOZ_ReportAssertionFailure|hg:hg.mozilla.org/mozilla-central:mfbt/Assertions.h:164

Can't tell whether DOM or widget is at fault here, but either way, this is not XPCOM.

I can hit this on a debug build when selecting stuff in https://listenfestival.be/, fwiw.