diff --git a/xpcom/threads/LazyIdleThread.cpp b/xpcom/threads/LazyIdleThread.cpp
index fb8d3451e40..9b40e27243e 100644
--- a/xpcom/threads/LazyIdleThread.cpp
+++ b/xpcom/threads/LazyIdleThread.cpp
@@ -11,7 +11,6 @@
 #include "nsComponentManagerUtils.h"
 #include "nsServiceManagerUtils.h"
 #include "nsThreadUtils.h"
-#include "mozilla/DebugOnly.h"
 #include "mozilla/Services.h"
 
 #ifdef DEBUG
@@ -235,17 +234,6 @@ LazyIdleThread::ShutdownThread()
 
   nsresult rv;
 
-  // Do this up front so that when we spin the event loop to shutdown the
-  // thread the idle timer doesn't cause us to reenter.
-  if (mIdleTimer) {
-    DebugOnly<nsresult> rv =
-      mIdleTimer->Cancel();
-    // nsTimerImpl::Cancel is infallible.
-    MOZ_ASSERT(NS_SUCCEEDED(rv));
-
-    mIdleTimer = nullptr;
-  }
-
   if (mThread) {
     if (mShutdownMethod == AutomaticShutdown && NS_IsMainThread()) {
       nsCOMPtr<nsIObserverService> obs =
@@ -272,7 +260,7 @@ LazyIdleThread::ShutdownThread()
 
     nsCOMPtr<nsIRunnable> runnable =
       NS_NewRunnableMethod(this, &LazyIdleThread::CleanupThread);
-    MOZ_ASSERT(runnable);
+    NS_ENSURE_TRUE(runnable, NS_ERROR_FAILURE);
 
     PreDispatch();
 
@@ -301,6 +289,13 @@ LazyIdleThread::ShutdownThread()
     }
   }
 
+  if (mIdleTimer) {
+    rv = mIdleTimer->Cancel();
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    mIdleTimer = nullptr;
+  }
+
   // If our temporary queue has any runnables then we need to dispatch them.
   if (queuedRunnables.Length()) {
     // If the thread manager has gone away then these runnables will never run.