diff --git a/netwerk/protocol/http/ASpdySession.cpp b/netwerk/protocol/http/ASpdySession.cpp
index f317d67cb7d..ba3b373f715 100644
--- a/netwerk/protocol/http/ASpdySession.cpp
+++ b/netwerk/protocol/http/ASpdySession.cpp
@@ -67,6 +67,8 @@ ASpdySession::NewSpdySession(uint32_t version,
 
 SpdyInformation::SpdyInformation()
 {
+  // highest index of enabled protocols is the
+  // most preferred for ALPN negotiaton
   Version[0] = SPDY_VERSION_3;
   VersionString[0] = NS_LITERAL_CSTRING("spdy/3");
 
diff --git a/netwerk/protocol/http/nsHttpConnection.cpp b/netwerk/protocol/http/nsHttpConnection.cpp
index 89b82fbbe69..b4b6ba701ab 100644
--- a/netwerk/protocol/http/nsHttpConnection.cpp
+++ b/netwerk/protocol/http/nsHttpConnection.cpp
@@ -482,17 +482,20 @@ nsHttpConnection::SetupNPNList(nsISSLSocketControl *ssl, uint32_t caps)
 
     // The first protocol is used as the fallback if none of the
     // protocols supported overlap with the server's list.
-    // In the case of overlap, matching priority is driven by
-    // the order of the server's advertisement.
+    // When using ALPN the advertised preferences are protocolArray indicies
+    // {1, .., N, 0} in decreasing order.
+    // For NPN, In the case of overlap, matching priority is driven by
+    // the order of the server's advertisement - with index 0 used when
+    // there is no match.
     protocolArray.AppendElement(NS_LITERAL_CSTRING("http/1.1"));
 
     if (gHttpHandler->IsSpdyEnabled() &&
         !(caps & NS_HTTP_DISALLOW_SPDY)) {
         LOG(("nsHttpConnection::SetupSSL Allow SPDY NPN selection"));
-        for (uint32_t index = 0; index < SpdyInformation::kCount; ++index) {
-            if (gHttpHandler->SpdyInfo()->ProtocolEnabled(index))
+        for (uint32_t index = SpdyInformation::kCount; index > 0; --index) {
+            if (gHttpHandler->SpdyInfo()->ProtocolEnabled(index - 1))
                 protocolArray.AppendElement(
-                    gHttpHandler->SpdyInfo()->VersionString[index]);
+                    gHttpHandler->SpdyInfo()->VersionString[index - 1]);
         }
     }