diff --git a/layout/style/Loader.cpp b/layout/style/Loader.cpp
index a58fecbf6e19..bbcdbda1c705 100644
--- a/layout/style/Loader.cpp
+++ b/layout/style/Loader.cpp
@@ -472,7 +472,7 @@ SheetLoadData::ScheduleLoadEventIfNeeded(nsresult aStatus)
 
   mStatus = aStatus;
 
-  nsCOMPtr<nsIThread> thread = do_GetMainThread();
+  nsCOMPtr<nsIThread> thread = do_GetCurrentThread();
   nsCOMPtr<nsIThreadInternal> internalThread = do_QueryInterface(thread);
   if (NS_SUCCEEDED(internalThread->AddObserver(this))) {
     // Make sure to block onload here
diff --git a/xpcom/threads/SynchronizedEventQueue.cpp b/xpcom/threads/SynchronizedEventQueue.cpp
new file mode 100644
index 000000000000..f91546d407e0
--- /dev/null
+++ b/xpcom/threads/SynchronizedEventQueue.cpp
@@ -0,0 +1,31 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "SynchronizedEventQueue.h"
+
+using namespace mozilla;
+
+void
+SynchronizedEventQueue::AddObserver(nsIThreadObserver* aObserver)
+{
+  MOZ_ASSERT(aObserver);
+  MOZ_ASSERT(!mEventObservers.Contains(aObserver));
+  mEventObservers.AppendElement(aObserver);
+}
+
+void
+SynchronizedEventQueue::RemoveObserver(nsIThreadObserver* aObserver)
+{
+  MOZ_ASSERT(aObserver);
+  MOZ_ALWAYS_TRUE(mEventObservers.RemoveElement(aObserver));
+}
+
+const nsTObserverArray<nsCOMPtr<nsIThreadObserver>>&
+SynchronizedEventQueue::EventObservers()
+{
+  return mEventObservers;
+}
+
diff --git a/xpcom/threads/SynchronizedEventQueue.h b/xpcom/threads/SynchronizedEventQueue.h
index f54f5131c4f7..d37c4d33b379 100644
--- a/xpcom/threads/SynchronizedEventQueue.h
+++ b/xpcom/threads/SynchronizedEventQueue.h
@@ -10,6 +10,7 @@
 #include "mozilla/AlreadyAddRefed.h"
 #include "mozilla/AbstractEventQueue.h"
 #include "mozilla/Mutex.h"
+#include "nsTObserverArray.h"
 
 class nsIThreadObserver;
 
@@ -65,6 +66,10 @@ public:
   virtual already_AddRefed<nsIThreadObserver> GetObserverOnThread() = 0;
   virtual void SetObserver(nsIThreadObserver* aObserver) = 0;
 
+  void AddObserver(nsIThreadObserver* aObserver);
+  void RemoveObserver(nsIThreadObserver* aObserver);
+  const nsTObserverArray<nsCOMPtr<nsIThreadObserver>>& EventObservers();
+
   virtual void EnableInputEventPrioritization() = 0;
   virtual void FlushInputEventPrioritization() = 0;
   virtual void SuspendInputEventPrioritization() = 0;
@@ -72,6 +77,9 @@ public:
 
 protected:
   virtual ~SynchronizedEventQueue() {}
+
+private:
+  nsTObserverArray<nsCOMPtr<nsIThreadObserver>> mEventObservers;
 };
 
 } // namespace mozilla
diff --git a/xpcom/threads/moz.build b/xpcom/threads/moz.build
index ba6d50ee4f10..2c5025e225c8 100644
--- a/xpcom/threads/moz.build
+++ b/xpcom/threads/moz.build
@@ -94,6 +94,7 @@ UNIFIED_SOURCES += [
     'RWLock.cpp',
     'SchedulerGroup.cpp',
     'SharedThreadPool.cpp',
+    'SynchronizedEventQueue.cpp',
     'SystemGroup.cpp',
     'TaskQueue.cpp',
     'ThreadEventQueue.cpp',
diff --git a/xpcom/threads/nsThread.cpp b/xpcom/threads/nsThread.cpp
index da90f7c0fcdc..8d71493a8992 100755
--- a/xpcom/threads/nsThread.cpp
+++ b/xpcom/threads/nsThread.cpp
@@ -874,11 +874,11 @@ void canary_alarm_handler(int signum)
 
 #endif
 
-#define NOTIFY_EVENT_OBSERVERS(func_, params_)                                 \
+#define NOTIFY_EVENT_OBSERVERS(observers_, func_, params_)                     \
   do {                                                                         \
-    if (!mEventObservers.IsEmpty()) {                                          \
-      nsAutoTObserverArray<NotNull<nsCOMPtr<nsIThreadObserver>>, 2>::ForwardIterator \
-        iter_(mEventObservers);                                                \
+    if (!observers_.IsEmpty()) {                                               \
+      nsTObserverArray<nsCOMPtr<nsIThreadObserver>>::ForwardIterator           \
+        iter_(observers_);                                                     \
       nsCOMPtr<nsIThreadObserver> obs_;                                        \
       while (iter_.HasMore()) {                                                \
         obs_ = iter_.GetNext();                                                \
@@ -951,7 +951,7 @@ nsThread::ProcessNextEvent(bool aMayWait, bool* aResult)
     obs->OnProcessNextEvent(this, reallyWait);
   }
 
-  NOTIFY_EVENT_OBSERVERS(OnProcessNextEvent, (this, reallyWait));
+  NOTIFY_EVENT_OBSERVERS(EventQueue()->EventObservers(), OnProcessNextEvent, (this, reallyWait));
 
 #ifdef MOZ_CANARY
   Canary canary;
@@ -1039,7 +1039,7 @@ nsThread::ProcessNextEvent(bool aMayWait, bool* aResult)
     }
   }
 
-  NOTIFY_EVENT_OBSERVERS(AfterProcessNextEvent, (this, *aResult));
+  NOTIFY_EVENT_OBSERVERS(EventQueue()->EventObservers(), AfterProcessNextEvent, (this, *aResult));
 
   if (obs) {
     obs->AfterProcessNextEvent(this, *aResult);
@@ -1146,13 +1146,7 @@ nsThread::AddObserver(nsIThreadObserver* aObserver)
     return NS_ERROR_NOT_SAME_THREAD;
   }
 
-  NS_WARNING_ASSERTION(!mEventObservers.Contains(aObserver),
-                       "Adding an observer twice!");
-
-  if (!mEventObservers.AppendElement(WrapNotNull(aObserver))) {
-    NS_WARNING("Out of memory!");
-    return NS_ERROR_OUT_OF_MEMORY;
-  }
+  EventQueue()->AddObserver(aObserver);
 
   return NS_OK;
 }
@@ -1164,9 +1158,7 @@ nsThread::RemoveObserver(nsIThreadObserver* aObserver)
     return NS_ERROR_NOT_SAME_THREAD;
   }
 
-  if (aObserver && !mEventObservers.RemoveElement(aObserver)) {
-    NS_WARNING("Removing an observer that was never added!");
-  }
+  EventQueue()->RemoveObserver(aObserver);
 
   return NS_OK;
 }