--- a/dom/base/TabGroup.cpp
+++ b/dom/base/TabGroup.cpp
@@ -48,23 +48,21 @@ TabGroup::~TabGroup()
 
 void
 TabGroup::EnsureThrottledEventQueue()
 {
   if (mThrottledEventQueue) {
     return;
   }
 
-  nsCOMPtr<nsIThread> mainThread;
-  NS_GetMainThread(getter_AddRefs(mainThread));
-  MOZ_DIAGNOSTIC_ASSERT(mainThread);
+  nsCOMPtr<nsIEventTarget> target = CreateEventTarget("ThrottledQueue", TaskCategory::Other);
 
   // This may return nullptr during xpcom shutdown.  This is ok as we
   // do not guarantee a ThrottledEventQueue will be present.
-  mThrottledEventQueue = ThrottledEventQueue::Create(mainThread);
+  mThrottledEventQueue = ThrottledEventQueue::Create(target);
 }
 
 TabGroup*
 TabGroup::GetChromeTabGroup()
 {
   if (!sChromeTabGroup) {
     sChromeTabGroup = new TabGroup(true /* chrome tab group */);
     ClearOnShutdown(&sChromeTabGroup);
@@ -133,24 +131,37 @@ TabGroup::AddDocument(const nsACString& 
 TabGroup::Join(nsPIDOMWindowOuter* aWindow, TabGroup* aTabGroup)
 {
   RefPtr<TabGroup> tabGroup = aTabGroup;
   if (!tabGroup) {
     tabGroup = new TabGroup();
   }
   MOZ_ASSERT(!tabGroup->mWindows.Contains(aWindow));
   tabGroup->mWindows.AppendElement(aWindow);
+
+  // Assert we have a throttled event queue. We null it out in Leave, but we
+  // should never have a Join after the last window Leaves.
+  MOZ_ASSERT(tabGroup->mThrottledEventQueue);
   return tabGroup.forget();
 }
 
 void
 TabGroup::Leave(nsPIDOMWindowOuter* aWindow)
 {
   MOZ_ASSERT(mWindows.Contains(aWindow));
   mWindows.RemoveElement(aWindow);
+
+  if (mWindows.IsEmpty()) {
+    // There is a RefPtr cycle TabGroup -> ThrottledEventQueue ->
+    // DispatcherEventTarget -> TabGroup. To avoid leaks, we need to break the
+    // chain somewhere. We shouldn't be using the ThrottledEventQueue for this
+    // TabGroup when no windows belong to it, so it's safe to null out the queue
+    // here.
+    mThrottledEventQueue = nullptr;
+  }
 }
 
 nsresult
 TabGroup::FindItemWithName(const nsAString& aName,
                            nsIDocShellTreeItem* aRequestor,
                            nsIDocShellTreeItem* aOriginalRequestor,
                            nsIDocShellTreeItem** aFoundItem)
 {