diff --git a/xpcom/glue/MainThreadUtils.h b/xpcom/glue/MainThreadUtils.h
deleted file mode 100644
index c1d24011b72a..000000000000
--- a/xpcom/glue/MainThreadUtils.h
+++ /dev/null
@@ -1,50 +0,0 @@
-/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* vim:set ts=2 sw=2 sts=2 et cindent: */
-/* 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/. */
-
-#ifndef MainThreadUtils_h_
-#define MainThreadUtils_h_
-
-#include "nscore.h"
-#include "mozilla/threads/nsThreadIDs.h"
-
-class nsIThread;
-
-/**
- * Get a reference to the main thread.
- *
- * @param result
- *   The resulting nsIThread object.
- */
-extern NS_COM_GLUE NS_METHOD
-NS_GetMainThread(nsIThread **result);
-
-#if defined(MOZILLA_INTERNAL_API) && defined(XP_WIN)
-bool NS_IsMainThread();
-#elif defined(MOZILLA_INTERNAL_API) && defined(NS_TLS)
-// This is defined in nsThreadManager.cpp and initialized to `Main` for the
-// main thread by nsThreadManager::Init.
-extern NS_TLS mozilla::threads::ID gTLSThreadID;
-#ifdef MOZ_ASAN
-// Temporary workaround, see bug 895845
-MOZ_ASAN_BLACKLIST static
-#else
-inline
-#endif
-bool NS_IsMainThread()
-{
-  return gTLSThreadID == mozilla::threads::Main;
-}
-#else
-/**
- * Test to see if the current thread is the main thread.
- *
- * @returns true if the current thread is the main thread, and false
- * otherwise.
- */
-extern NS_COM_GLUE bool NS_IsMainThread();
-#endif
-
-#endif // MainThreadUtils_h_
diff --git a/xpcom/glue/moz.build b/xpcom/glue/moz.build
index c79904650ae3..ff9e3ebdf98a 100644
--- a/xpcom/glue/moz.build
+++ b/xpcom/glue/moz.build
@@ -14,7 +14,6 @@ if CONFIG['OS_ARCH'] == 'WINNT':
 MODULE = 'xpcom'
 
 EXPORTS += [
-    'MainThreadUtils.h',
     'nsArrayEnumerator.h',
     'nsArrayUtils.h',
     'nsBaseHashtable.h',
diff --git a/xpcom/glue/nsThreadUtils.h b/xpcom/glue/nsThreadUtils.h
index e53cc013153c..82a555766c53 100644
--- a/xpcom/glue/nsThreadUtils.h
+++ b/xpcom/glue/nsThreadUtils.h
@@ -9,7 +9,7 @@
 
 #include "prthread.h"
 #include "prinrval.h"
-#include "MainThreadUtils.h"
+#include "nscore.h"
 #include "nsIThreadManager.h"
 #include "nsIThread.h"
 #include "nsIRunnable.h"
@@ -17,6 +17,7 @@
 #include "nsStringGlue.h"
 #include "nsCOMPtr.h"
 #include "nsAutoPtr.h"
+#include "mozilla/threads/nsThreadIDs.h"
 #include "mozilla/Likely.h"
 
 //-----------------------------------------------------------------------------
@@ -90,6 +91,41 @@ NS_NewNamedThread(const char (&name)[LEN],
 extern NS_COM_GLUE NS_METHOD
 NS_GetCurrentThread(nsIThread **result);
 
+/**
+ * Get a reference to the main thread.
+ *
+ * @param result
+ *   The resulting nsIThread object.
+ */
+extern NS_COM_GLUE NS_METHOD
+NS_GetMainThread(nsIThread **result);
+
+#if defined(MOZILLA_INTERNAL_API) && defined(XP_WIN)
+bool NS_IsMainThread();
+#elif defined(MOZILLA_INTERNAL_API) && defined(NS_TLS)
+// This is defined in nsThreadManager.cpp and initialized to `Main` for the
+// main thread by nsThreadManager::Init.
+extern NS_TLS mozilla::threads::ID gTLSThreadID;
+#ifdef MOZ_ASAN
+// Temporary workaround, see bug 895845
+MOZ_ASAN_BLACKLIST static
+#else
+inline
+#endif
+bool NS_IsMainThread()
+{
+  return gTLSThreadID == mozilla::threads::Main;
+}
+#else
+/**
+ * Test to see if the current thread is the main thread.
+ *
+ * @returns true if the current thread is the main thread, and false
+ * otherwise.
+ */
+extern NS_COM_GLUE bool NS_IsMainThread();
+#endif
+
 /**
  * Dispatch the given event to the current thread.
  *