new file mode 100644
--- /dev/null
+++ b/xpcom/tests/gtest/TestMultiplexInputStream.cpp
@@ -0,0 +1,101 @@
+/* -*- 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 "gtest/gtest.h"
+#include "nsComponentManagerUtils.h"
+#include "nsCOMPtr.h"
+#include "nsIInputStream.h"
+#include "nsIMultiplexInputStream.h"
+#include "nsISeekableStream.h"
+#include "nsStringStream.h"
+
+TEST(MultiplexInputStream, Seek_SET)
+{
+  nsCString buf1;
+  nsCString buf2;
+  nsCString buf3;
+  buf1.Assign("Hello world");
+  buf2.Assign("The quick brown fox jumped over the lazy dog");
+  buf3.Assign("Foo bar");
+
+  nsCOMPtr<nsIInputStream> inputStream1;
+  nsCOMPtr<nsIInputStream> inputStream2;
+  nsCOMPtr<nsIInputStream> inputStream3;
+  nsresult rv = NS_NewCStringInputStream(getter_AddRefs(inputStream1), buf1);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  rv = NS_NewCStringInputStream(getter_AddRefs(inputStream2), buf2);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  rv = NS_NewCStringInputStream(getter_AddRefs(inputStream3), buf3);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+
+  nsCOMPtr<nsIMultiplexInputStream> multiplexStream =
+    do_CreateInstance("@mozilla.org/io/multiplex-input-stream;1");
+  ASSERT_TRUE(multiplexStream);
+
+  rv = multiplexStream->AppendStream(inputStream1);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  rv = multiplexStream->AppendStream(inputStream2);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  rv = multiplexStream->AppendStream(inputStream3);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+
+  uint64_t length;
+  uint32_t count;
+  char readBuf[4096];
+  nsCOMPtr<nsISeekableStream> seekStream = do_QueryInterface(multiplexStream);
+  ASSERT_TRUE(seekStream);
+
+  // Seek forward in first input stream
+  rv = seekStream->Seek(nsISeekableStream::NS_SEEK_SET, 1);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  rv = multiplexStream->Available(&length);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)buf1.Length() + buf2.Length() + buf3.Length() - 1,
+            length);
+
+  // Check read is correct
+  rv = multiplexStream->Read(readBuf, 3, &count);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)3, count);
+  rv = multiplexStream->Available(&length);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)buf1.Length() + buf2.Length() + buf3.Length() - 4,
+            length);
+  ASSERT_EQ(0, strncmp(readBuf, "ell", count));
+
+  // Seek to start of third input stream
+  rv = seekStream->Seek(nsISeekableStream::NS_SEEK_SET,
+                        buf1.Length() + buf2.Length());
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  rv = multiplexStream->Available(&length);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)buf3.Length(), length);
+
+  // Check read is correct
+  rv = multiplexStream->Read(readBuf, 5, &count);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)5, count);
+  rv = multiplexStream->Available(&length);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)buf3.Length() - 5, length);
+  ASSERT_EQ(0, strncmp(readBuf, "Foo b", count));
+
+  // Seek back to start of second stream (covers bug 1272371)
+  rv = seekStream->Seek(nsISeekableStream::NS_SEEK_SET, buf1.Length());
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  rv = multiplexStream->Available(&length);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)buf2.Length() + buf3.Length(), length);
+
+  // Check read is correct
+  rv = multiplexStream->Read(readBuf, 6, &count);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)6, count);
+  rv = multiplexStream->Available(&length);
+  ASSERT_TRUE(NS_SUCCEEDED(rv));
+  ASSERT_EQ((uint64_t)buf2.Length() - 6 + buf3.Length(), length);
+  ASSERT_EQ(0, strncmp(readBuf, "The qu", count));
+}
\ No newline at end of file

--- a/xpcom/tests/gtest/moz.build
+++ b/xpcom/tests/gtest/moz.build
@@ -16,16 +16,17 @@ UNIFIED_SOURCES += [
     'TestCOMPtrEq.cpp',
     'TestCRT.cpp',
     'TestEncoding.cpp',
     'TestEscapeURL.cpp',
     'TestExpirationTracker.cpp',
     'TestFile.cpp',
     'TestGCPostBarriers.cpp',
     'TestID.cpp',
+    'TestMultiplexInputStream.cpp',
     'TestNsDeque.cpp',
     'TestNSPRLogModulesParser.cpp',
     'TestObserverArray.cpp',
     'TestObserverService.cpp',
     'TestPipes.cpp',
     'TestPLDHash.cpp',
     'TestPriorityQueue.cpp',
     'TestRacingServiceManager.cpp',