diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index ecf29bef53..1f0ad89f71 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1602,7 +1602,7 @@ pref("network.http.spdy.coalesce-hostnames", true);
 pref("network.http.spdy.persistent-settings", false);
 pref("network.http.spdy.ping-threshold", 58);
 pref("network.http.spdy.ping-timeout", 8);
-pref("network.http.spdy.send-buffer-size", 131072);
+pref("network.http.spdy.send-buffer-size", 0); // 0 - Auto (managed by OS)
 pref("network.http.spdy.allow-push", true);
 pref("network.http.spdy.push-allowance", 131072);   // 128KB
 pref("network.http.spdy.pull-allowance", 12582912); // 12MB
diff --git a/netwerk/protocol/http/Http2Stream.cpp b/netwerk/protocol/http/Http2Stream.cpp
index 9b8f13c2d2..79942af07d 100644
--- a/netwerk/protocol/http/Http2Stream.cpp
+++ b/netwerk/protocol/http/Http2Stream.cpp
@@ -788,8 +788,9 @@ Http2Stream::UpdateTransportSendEvents(uint32_t count)
   // the session and cap the send buffers by default at 128KB.
   // (10Mbit/sec @ 100ms)
   //
+  // This feature is disabled by default.
   uint32_t bufferSize = gHttpHandler->SpdySendBufferSize();
-  if ((mTotalSent > bufferSize) && !mSetTCPSocketBuffer) {
+  if ((bufferSize > 0) && (mTotalSent > bufferSize) && !mSetTCPSocketBuffer) {
     mSetTCPSocketBuffer = 1;
     mSocketTransport->SetSendBufferSize(bufferSize);
   }
diff --git a/netwerk/protocol/http/nsHttpHandler.cpp b/netwerk/protocol/http/nsHttpHandler.cpp
index 3a5ec2936c..4e96415105 100644
--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -1437,8 +1437,12 @@ nsHttpHandler::PrefsChanged(nsIPrefBranch *prefs, const char *pref)
     // closing the session.
     if (PREF_CHANGED(HTTP_PREF("spdy.send-buffer-size"))) {
         rv = prefs->GetIntPref(HTTP_PREF("spdy.send-buffer-size"), &val);
-        if (NS_SUCCEEDED(rv))
-            mSpdySendBufferSize = (uint32_t) clamped(val, 1500, 0x7fffffff);
+        if (NS_SUCCEEDED(rv)) {
+            if (val != 0)
+                mSpdySendBufferSize = (uint32_t) clamped(val, 1500, 0x7fffffff);
+            else
+                mSpdySendBufferSize = 0;
+        }
     }
 
     // The maximum amount of time to wait for socket transport to be