This could be related to (or be the cause of) bug 1320170. See the red bc jobs on https://treeherder.mozilla.org/#/jobs?repo=try&revision=587f39750630bc567afd3f2b5113218ff53a12e9 which is the try push for my screensharing test (bug 1313324) 15:18:45 INFO - Assertion failure: aInstanceSize == 0 || entry->GetClassSize() == aInstanceSize (Mismatched sizes were recorded in the memory leak logging table. The usual cause of this is having a templated class that uses MOZ_COUNT_{C,D}TOR in the constructor or destructor, respectively. As a workaround, the MOZ_COUNT_{C,D}TOR calls can be moved to a non-templated base class.), at /builds/slave/try-m64-d-00000000000000000000/build/src/xpcom/base/nsTraceRefcnt.cpp:441 15:18:45 INFO - #01: mozilla::MediaManager::GetNonE10sParent() [dom/media/MediaManager.cpp:1846] 15:18:45 INFO - 15:18:45 INFO - #02: mozilla::media::GetOriginKey(nsCString const&, bool, bool) [dom/media/systemservices/MediaChild.cpp:32] 15:18:45 INFO - 15:18:45 INFO - #03: mozilla::MediaManager::EnumerateDevicesImpl(unsigned long long, mozilla::dom::MediaSourceEnum, mozilla::dom::MediaSourceEnum, bool) [mfbt/AlreadyAddRefed.h:116] 15:18:45 INFO - 15:18:45 INFO - #04: mozilla::MediaManager::GetUserMedia(nsPIDOMWindowInner*, mozilla::dom::MediaStreamConstraints const&, nsIDOMGetUserMediaSuccessCallback*, nsIDOMGetUserMediaErrorCallback*) [mfbt/AlreadyAddRefed.h:116] 15:18:45 INFO - 15:18:45 INFO - #05: mozilla::dom::MediaDevices::GetUserMedia(mozilla::dom::MediaStreamConstraints const&, mozilla::ErrorResult&) [dom/bindings/ErrorResult.h:317]