--- a/netwerk/protocol/http/nsHttpAtomList.h
+++ b/netwerk/protocol/http/nsHttpAtomList.h
@@ -72,16 +72,17 @@ HTTP_ATOM(Proxy_Connection,          "Pr
 HTTP_ATOM(Range,                     "Range")
 HTTP_ATOM(Referer,                   "Referer")
 HTTP_ATOM(Retry_After,               "Retry-After")
 HTTP_ATOM(Server,                    "Server")
 HTTP_ATOM(Service_Worker_Allowed,    "Service-Worker-Allowed")
 HTTP_ATOM(Set_Cookie,                "Set-Cookie")
 HTTP_ATOM(Set_Cookie2,               "Set-Cookie2")
 HTTP_ATOM(Status_URI,                "Status-URI")
+HTTP_ATOM(Strict_Transport_Security, "Strict-Transport-Security")
 HTTP_ATOM(TE,                        "TE")
 HTTP_ATOM(Title,                     "Title")
 HTTP_ATOM(Timeout,                   "Timeout")
 HTTP_ATOM(Trailer,                   "Trailer")
 HTTP_ATOM(Transfer_Encoding,         "Transfer-Encoding")
 HTTP_ATOM(URI,                       "URI")
 HTTP_ATOM(Upgrade,                   "Upgrade")
 HTTP_ATOM(User_Agent,                "User-Agent")

--- a/netwerk/protocol/http/nsHttpHeaderArray.cpp
+++ b/netwerk/protocol/http/nsHttpHeaderArray.cpp
@@ -49,17 +49,17 @@ nsHttpHeaderArray::SetHeader(nsHttpAtom 
     }
 
     MOZ_ASSERT(!entry || variety != eVarietyRequestDefault,
                "Cannot set default entry which overrides existing entry!");
     if (!entry) {
         return SetHeader_internal(header, value, variety);
     } else if (merge && !IsSingletonHeader(header)) {
         return MergeHeader(header, entry, value, variety);
-    } else {
+    } else if (!IsIgnoreMultipleHeader(header)) {
         // Replace the existing string with the new value
         if (entry->variety == eVarietyResponseNetOriginalAndResponse) {
             MOZ_ASSERT(variety == eVarietyResponse);
             entry->variety = eVarietyResponseNetOriginal;
             return SetHeader_internal(header, value, variety);
         } else {
             entry->value = value;
             entry->variety = variety;
@@ -146,17 +146,17 @@ nsHttpHeaderArray::SetHeaderFromNet(nsHt
         if (NS_FAILED(rv)) {
             return rv;
         }
         if (response) {
             rv = SetHeader_internal(header, value,
                                     eVarietyResponseNetOriginal);
         }
         return rv;
-    } else {
+    } else if (!IsIgnoreMultipleHeader(header)) {
         // Multiple instances of non-mergeable header received from network
         // - ignore if same value
         if (!entry->value.Equals(value)) {
             if (IsSuspectDuplicateHeader(header)) {
                 // reply may be corrupt/hacked (ex: CLRF injection attacks)
                 return NS_ERROR_CORRUPTED_CONTENT;
             } // else silently drop value: keep value from 1st header seen
             LOG(("Header %s silently dropped as non mergeable header\n",

--- a/netwerk/protocol/http/nsHttpHeaderArray.h
+++ b/netwerk/protocol/http/nsHttpHeaderArray.h
@@ -146,16 +146,18 @@ private:
                                       const nsACString &value,
                                       HeaderVariety variety);
     MOZ_MUST_USE nsresult SetHeader_internal(nsHttpAtom header,
                                              const nsACString &value,
                                              HeaderVariety variety);
 
     // Header cannot be merged: only one value possible
     bool    IsSingletonHeader(nsHttpAtom header);
+    // Header cannot be merged, and subsequent values should be ignored
+    bool    IsIgnoreMultipleHeader(nsHttpAtom header);
     // For some headers we want to track empty values to prevent them being
     // combined with non-empty ones as a CRLF attack vector
     bool    TrackEmptyHeader(nsHttpAtom header);
 
     // Subset of singleton headers: should never see multiple, different
     // instances of these, else something fishy may be going on (like CLRF
     // injection)
     bool    IsSuspectDuplicateHeader(nsHttpAtom header);
@@ -217,17 +219,32 @@ nsHttpHeaderArray::IsSingletonHeader(nsH
            header == nsHttp::Referer                     ||
            header == nsHttp::Host                        ||
            header == nsHttp::Authorization               ||
            header == nsHttp::Proxy_Authorization         ||
            header == nsHttp::If_Modified_Since           ||
            header == nsHttp::If_Unmodified_Since         ||
            header == nsHttp::From                        ||
            header == nsHttp::Location                    ||
-           header == nsHttp::Max_Forwards;
+           header == nsHttp::Max_Forwards                ||
+           // Ignore-multiple-headers are singletons in the sense that they
+           // shouldn't be merged.
+           IsIgnoreMultipleHeader(header);
+}
+
+// These are headers for which, in the presence of multiple values, we only
+// consider the first.
+inline bool nsHttpHeaderArray::IsIgnoreMultipleHeader(nsHttpAtom header)
+{
+    // https://tools.ietf.org/html/rfc6797#section-8:
+    //
+    //     If a UA receives more than one STS header field in an HTTP
+    //     response message over secure transport, then the UA MUST process
+    //     only the first such header field.
+    return header == nsHttp::Strict_Transport_Security;
 }
 
 inline bool
 nsHttpHeaderArray::TrackEmptyHeader(nsHttpAtom header)
 {
     return header == nsHttp::Content_Length ||
            header == nsHttp::Location ||
            header == nsHttp::Access_Control_Allow_Origin;

new file mode 100644
--- /dev/null
+++ b/netwerk/test/gtest/TestHeaders.cpp
@@ -0,0 +1,29 @@
+#include "gtest/gtest.h"
+
+#include "nsHttpHeaderArray.h"
+
+
+TEST(TestHeaders, DuplicateHSTS) {
+    // When the Strict-Transport-Security header is sent multiple times, its
+    // effective value is the value of the first item. It is not merged as other
+    // headers are.
+    mozilla::net::nsHttpHeaderArray headers;
+    nsresult rv = headers.SetHeaderFromNet(
+        mozilla::net::nsHttp::Strict_Transport_Security, NS_LITERAL_CSTRING("max-age=360"), true
+    );
+    ASSERT_EQ(rv, NS_OK);
+
+    nsAutoCString h;
+    rv = headers.GetHeader(mozilla::net::nsHttp::Strict_Transport_Security, h);
+    ASSERT_EQ(rv, NS_OK);
+    ASSERT_EQ(h.get(), "max-age=360");
+
+    rv = headers.SetHeaderFromNet(
+        mozilla::net::nsHttp::Strict_Transport_Security, NS_LITERAL_CSTRING("max-age=720"), true
+    );
+    ASSERT_EQ(rv, NS_OK);
+
+    rv = headers.GetHeader(mozilla::net::nsHttp::Strict_Transport_Security, h);
+    ASSERT_EQ(rv, NS_OK);
+    ASSERT_EQ(h.get(), "max-age=360");
+}

--- a/netwerk/test/gtest/moz.build
+++ b/netwerk/test/gtest/moz.build
@@ -1,15 +1,16 @@
 # -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
 # vim: set filetype=python:
 # 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/.
 
 UNIFIED_SOURCES += [
+    'TestHeaders.cpp',
     'TestHttpAuthUtils.cpp',
     'TestProtocolProxyService.cpp',
     'TestStandardURL.cpp',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul-gtest'