--- a/dom/security/test/hsts/head.js
+++ b/dom/security/test/hsts/head.js
@@ -267,17 +267,20 @@ function SetupPrefTestEnvironment(which,
 
   var prefs = [["security.mixed_content.block_active_content",
                 settings.block_active],
                ["security.mixed_content.block_display_content",
                 settings.block_display],
                ["security.mixed_content.use_hsts",
                 settings.use_hsts],
                ["security.mixed_content.send_hsts_priming",
-                settings.send_hsts_priming]];
+                settings.send_hsts_priming],
+               ["security.mixed_content.hsts_priming_request_timeout",
+                30000],
+  ];
 
   if (additional_prefs) {
     for (let idx in additional_prefs) {
       prefs.push(additional_prefs[idx]);
     }
   }
 
   console.log("prefs=%s", prefs);

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5527,18 +5527,21 @@ pref("security.mixed_content.send_hsts_p
 // Don't change the order of evaluation of mixed-content and HSTS upgrades in
 // order to be most compatible with current standards
 pref("security.mixed_content.use_hsts", false);
 #else
 // Change the order of evaluation so HSTS upgrades happen before
 // mixed-content blocking
 pref("security.mixed_content.use_hsts", true);
 #endif
-// Approximately 1 week default cache for HSTS priming failures
+// Approximately 1 week default cache for HSTS priming failures, in seconds
 pref ("security.mixed_content.hsts_priming_cache_timeout", 10080);
+// Force the channel to timeout in 3 seconds if we have not received
+// expects a time in milliseconds
+pref ("security.mixed_content.hsts_priming_request_timeout", 3000);
 
 // Disable Storage api in release builds.
 #ifdef NIGHTLY_BUILD
 pref("dom.storageManager.enabled", true);
 #else
 pref("dom.storageManager.enabled", false);
 #endif
 

--- a/netwerk/protocol/http/HSTSPrimerListener.cpp
+++ b/netwerk/protocol/http/HSTSPrimerListener.cpp
@@ -12,53 +12,81 @@
 #include "nsSecurityHeaderParser.h"
 #include "nsISiteSecurityService.h"
 #include "nsISocketProvider.h"
 #include "nsISSLStatus.h"
 #include "nsISSLStatusProvider.h"
 #include "nsStreamUtils.h"
 #include "nsHttpChannel.h"
 #include "LoadInfo.h"
+#include "mozilla/Unused.h"
 
 namespace mozilla {
 namespace net {
 
 using namespace mozilla;
 
 NS_IMPL_ISUPPORTS(HSTSPrimingListener, nsIStreamListener,
                   nsIRequestObserver, nsIInterfaceRequestor)
 
+// default to 3000ms, same as the preference
+uint32_t HSTSPrimingListener::sHSTSPrimingTimeout = 3000;
+
+
+HSTSPrimingListener::HSTSPrimingListener(nsIHstsPrimingCallback* aCallback)
+  : mCallback(aCallback)
+{
+  static nsresult rv =
+    Preferences::AddUintVarCache(&sHSTSPrimingTimeout,
+        "security.mixed_content.hsts_priming_request_timeout");
+  Unused << rv;
+}
+
 NS_IMETHODIMP
 HSTSPrimingListener::GetInterface(const nsIID & aIID, void **aResult)
 {
   return QueryInterface(aIID, aResult);
 }
 
-NS_IMETHODIMP
-HSTSPrimingListener::OnStartRequest(nsIRequest *aRequest,
-                                    nsISupports *aContext)
+void
+HSTSPrimingListener::ReportTiming(nsresult aResult)
 {
-  nsresult primingResult = CheckHSTSPrimingRequestStatus(aRequest);
-  nsCOMPtr<nsIHstsPrimingCallback> callback(mCallback);
-  mCallback = nullptr;
-
   nsCOMPtr<nsITimedChannel> timingChannel =
-    do_QueryInterface(callback);
+    do_QueryInterface(mCallback);
   if (timingChannel) {
     TimeStamp channelCreationTime;
     nsresult rv = timingChannel->GetChannelCreation(&channelCreationTime);
     if (NS_SUCCEEDED(rv) && !channelCreationTime.IsNull()) {
       PRUint32 interval =
         (PRUint32) (TimeStamp::Now() - channelCreationTime).ToMilliseconds();
       Telemetry::Accumulate(Telemetry::HSTS_PRIMING_REQUEST_DURATION,
-          (NS_SUCCEEDED(primingResult)) ? NS_LITERAL_CSTRING("success")
-                                        : NS_LITERAL_CSTRING("failure"),
+          (NS_SUCCEEDED(aResult)) ? NS_LITERAL_CSTRING("success")
+                                  : NS_LITERAL_CSTRING("failure"),
           interval);
     }
   }
+}
+
+NS_IMETHODIMP
+HSTSPrimingListener::OnStartRequest(nsIRequest *aRequest,
+                                    nsISupports *aContext)
+{
+  nsCOMPtr<nsIHstsPrimingCallback> callback;
+  callback.swap(mCallback);
+  if (mHSTSPrimingTimer) {
+    Unused << mHSTSPrimingTimer->Cancel();
+    mHSTSPrimingTimer = nullptr;
+  }
+
+  // if callback is null, we have already canceled this request and reported
+  // the failure
+  NS_ENSURE_STATE(callback);
+
+  nsresult primingResult = CheckHSTSPrimingRequestStatus(aRequest);
+  ReportTiming(primingResult);
 
   if (NS_FAILED(primingResult)) {
     LOG(("HSTS Priming Failed (request was not approved)"));
     return callback->OnHSTSPrimingFailed(primingResult, false);
   }
 
   LOG(("HSTS Priming Succeeded (request was approved)"));
   return callback->OnHSTSPrimingSucceeded(false);
@@ -134,22 +162,47 @@ HSTSPrimingListener::OnDataAvailable(nsI
                                      nsIInputStream *inStr,
                                      uint64_t sourceOffset,
                                      uint32_t count)
 {
   uint32_t totalRead;
   return inStr->ReadSegments(NS_DiscardSegment, nullptr, count, &totalRead);
 }
 
+/** nsITimerCallback **/
+NS_IMETHODIMP
+HSTSPrimingListener::Notify(nsITimer* timer)
+{
+  nsresult rv;
+  nsCOMPtr<nsIHstsPrimingCallback> callback;
+  callback.swap(mCallback);
+  NS_ENSURE_STATE(callback);
+  ReportTiming(NS_ERROR_HSTS_PRIMING_TIMEOUT);
+
+  if (mPrimingChannel) {
+    rv = mPrimingChannel->Cancel(NS_ERROR_HSTS_PRIMING_TIMEOUT);
+    if (NS_FAILED(rv)) {
+      // do what?
+      NS_ERROR("HSTS Priming timed out, and we got an error canceling the priming channel.");
+    }
+  }
+
+  rv = callback->OnHSTSPrimingFailed(NS_ERROR_HSTS_PRIMING_TIMEOUT, false);
+  if (NS_FAILED(rv)) {
+    NS_ERROR("HSTS Priming timed out, and we got an error reporting the failure.");
+  }
+
+  return NS_OK; // unused
+}
+
 // static
 nsresult
 HSTSPrimingListener::StartHSTSPriming(nsIChannel* aRequestChannel,
                                       nsIHstsPrimingCallback* aCallback)
 {
-
   nsCOMPtr<nsIURI> finalChannelURI;
   nsresult rv = NS_GetFinalChannelURI(aRequestChannel, getter_AddRefs(finalChannelURI));
   NS_ENSURE_SUCCESS(rv, rv);
 
   nsCOMPtr<nsIURI> uri;
   rv = NS_GetSecureUpgradedURI(finalChannelURI, getter_AddRefs(uri));
   NS_ENSURE_SUCCESS(rv,rv);
 
@@ -225,16 +278,18 @@ HSTSPrimingListener::StartHSTSPriming(ns
   NS_ENSURE_SUCCESS(rv, rv);
 
   // Set method and headers
   nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(primingChannel);
   if (!httpChannel) {
     NS_ERROR("HSTSPrimingListener: Failed to QI to nsIHttpChannel!");
     return NS_ERROR_FAILURE;
   }
+  nsCOMPtr<nsIHttpChannelInternal> internal = do_QueryInterface(primingChannel);
+  NS_ENSURE_STATE(internal);
 
   // Currently using HEAD per the draft, but under discussion to change to GET
   // with credentials so if the upgrade is approved the result is already cached.
   rv = httpChannel->SetRequestMethod(NS_LITERAL_CSTRING("HEAD"));
   NS_ENSURE_SUCCESS(rv, rv);
 
   rv = httpChannel->
     SetRequestHeader(NS_LITERAL_CSTRING("Upgrade-Insecure-Requests"),
@@ -244,30 +299,51 @@ HSTSPrimingListener::StartHSTSPriming(ns
   // attempt to set the class of service flags on the new channel
   nsCOMPtr<nsIClassOfService> requestClass = do_QueryInterface(aRequestChannel);
   if (!requestClass) {
     NS_ERROR("HSTSPrimingListener: aRequestChannel is not an nsIClassOfService");
     return NS_ERROR_FAILURE;
   }
   nsCOMPtr<nsIClassOfService> primingClass = do_QueryInterface(httpChannel);
   if (!primingClass) {
-    NS_ERROR("HSTSPrimingListener: aRequestChannel is not an nsIClassOfService");
+    NS_ERROR("HSTSPrimingListener: httpChannel is not an nsIClassOfService");
     return NS_ERROR_FAILURE;
   }
 
   uint32_t classFlags = 0;
   rv = requestClass ->GetClassFlags(&classFlags);
   NS_ENSURE_SUCCESS(rv, rv);
   rv = primingClass->SetClassFlags(classFlags);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  // Set up listener which will start the original channel
-  nsCOMPtr<nsIStreamListener> primingListener(new HSTSPrimingListener(aCallback));
+  // The priming channel should have highest priority so that it completes as
+  // quickly as possible, allowing the load to proceed.
+  nsCOMPtr<nsISupportsPriority> p = do_QueryInterface(primingChannel);
+  if (p) {
+    uint32_t priority = nsISupportsPriority::PRIORITY_HIGHEST;
+
+    p->SetPriority(priority);
+  }
 
+  // Set up listener which will start the original channel
+  HSTSPrimingListener* listener = new HSTSPrimingListener(aCallback);
   // Start priming
-  rv = primingChannel->AsyncOpen2(primingListener);
+  rv = primingChannel->AsyncOpen2(listener);
   NS_ENSURE_SUCCESS(rv, rv);
+  listener->mPrimingChannel.swap(primingChannel);
+
+  nsCOMPtr<nsITimer> timer = do_CreateInstance(NS_TIMER_CONTRACTID);
+  NS_ENSURE_STATE(timer);
+
+  rv = timer->InitWithCallback(listener,
+                               sHSTSPrimingTimeout,
+                               nsITimer::TYPE_ONE_SHOT);
+  if (NS_FAILED(rv)) {
+    NS_ERROR("HSTS Priming failed to initialize channel cancellation timer");
+  }
+
+  listener->mHSTSPrimingTimer.swap(timer);
 
   return NS_OK;
 }
 
 } // namespace net
 } // namespace mozilla

--- a/netwerk/protocol/http/HSTSPrimerListener.h
+++ b/netwerk/protocol/http/HSTSPrimerListener.h
@@ -44,36 +44,41 @@ enum HSTSPrimingResult {
   // of mixed-content and hsts, and mixed-content blocks the load
   eHSTS_PRIMING_SUCCEEDED_BLOCK   = 5,
   // When priming succeeds, but preferences require preservation of the order
   // of mixed-content and hsts, and mixed-content allows the load over http
   eHSTS_PRIMING_SUCCEEDED_HTTP    = 6,
   // HSTS priming failed, and the load is blocked by mixed-content
   eHSTS_PRIMING_FAILED_BLOCK      = 7,
   // HSTS priming failed, and the load is allowed by mixed-content
-  eHSTS_PRIMING_FAILED_ACCEPT     = 8
+  eHSTS_PRIMING_FAILED_ACCEPT     = 8,
+  // The HSTS Priming request timed out, and the load is blocked by
+  // mixed-content
+  eHSTS_PRIMING_TIMEOUT_BLOCK     = 9,
+  // The HSTS Priming request timed out, and the load is allowed by
+  // mixed-content
+  eHSTS_PRIMING_TIMEOUT_ACCEPT    = 10
 };
 
 //////////////////////////////////////////////////////////////////////////
 // Class used as streamlistener and notification callback when
 // doing the HEAD request for an HSTS Priming check. Needs to be an
 // nsIStreamListener in order to receive events from AsyncOpen2
 class HSTSPrimingListener final : public nsIStreamListener,
-                                  public nsIInterfaceRequestor
+                                  public nsIInterfaceRequestor,
+                                  public nsITimerCallback
 {
 public:
-  explicit HSTSPrimingListener(nsIHstsPrimingCallback* aCallback)
-   : mCallback(aCallback)
-  {
-  }
+  explicit HSTSPrimingListener(nsIHstsPrimingCallback* aCallback);
 
   NS_DECL_ISUPPORTS
   NS_DECL_NSISTREAMLISTENER
   NS_DECL_NSIREQUESTOBSERVER
   NS_DECL_NSIINTERFACEREQUESTOR
+  NS_DECL_NSITIMERCALLBACK
 
 private:
   ~HSTSPrimingListener() {}
 
   // Only nsHttpChannel can invoke HSTS priming
   friend class mozilla::net::nsHttpChannel;
 
   /**
@@ -91,18 +96,38 @@ private:
                                    nsIHstsPrimingCallback* aCallback);
 
   /**
    * Given a request, return NS_OK if it has resulted in a cached HSTS update.
    * We don't need to check for the header as that has already been done for us.
    */
   nsresult CheckHSTSPrimingRequestStatus(nsIRequest* aRequest);
 
+  // send telemetry about how long HSTS priming requests take
+  void ReportTiming(nsresult aResult);
+
   /**
    * the nsIHttpChannel to notify with the result of HSTS priming.
    */
   nsCOMPtr<nsIHstsPrimingCallback> mCallback;
+
+  /**
+   * Keep a handle to the priming channel so we can cancel it on timeout
+   */
+  nsCOMPtr<nsIChannel> mPrimingChannel;
+
+  /**
+   * Keep a handle to the timer around so it can be canceled if we don't time
+   * out.
+   */
+  nsCOMPtr<nsITimer> mHSTSPrimingTimer;
+
+  /**
+   * How long (in ms) before an HSTS Priming channel times out.
+   * Preference: security.mixed_content.hsts_priming_request_timeout
+   */
+  static uint32_t sHSTSPrimingTimeout;
 };
 
 
 }} // mozilla::net
 
 #endif // HSTSPrimingListener_h__

--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -7984,29 +7984,34 @@ nsHttpChannel::OnHSTSPrimingSucceeded(bo
  */
 nsresult
 nsHttpChannel::OnHSTSPrimingFailed(nsresult aError, bool aCached)
 {
     bool wouldBlock = mLoadInfo->GetMixedContentWouldBlock();
 
     LOG(("HSTS Priming Failed [this=%p], %s the load", this,
                 (wouldBlock) ? "blocking" : "allowing"));
-    if (aCached) {
+    if (aError == NS_ERROR_HSTS_PRIMING_TIMEOUT) {
+        // A priming request was sent, but timed out
+        Telemetry::Accumulate(Telemetry::MIXED_CONTENT_HSTS_PRIMING_RESULT,
+                (wouldBlock) ?  HSTSPrimingResult::eHSTS_PRIMING_TIMEOUT_BLOCK :
+                HSTSPrimingResult::eHSTS_PRIMING_TIMEOUT_ACCEPT);
+    } else if (aCached) {
         // Between the time we marked for priming and started the priming request,
         // the host was found to not allow the upgrade, probably from another
         // priming request.
         Telemetry::Accumulate(Telemetry::MIXED_CONTENT_HSTS_PRIMING_RESULT,
                 (wouldBlock) ?  HSTSPrimingResult::eHSTS_PRIMING_CACHED_BLOCK :
-                                HSTSPrimingResult::eHSTS_PRIMING_CACHED_NO_UPGRADE);
+                HSTSPrimingResult::eHSTS_PRIMING_CACHED_NO_UPGRADE);
     } else {
         // A priming request was sent, and no HSTS header was found that allows
         // the upgrade.
         Telemetry::Accumulate(Telemetry::MIXED_CONTENT_HSTS_PRIMING_RESULT,
                 (wouldBlock) ?  HSTSPrimingResult::eHSTS_PRIMING_FAILED_BLOCK :
-                                HSTSPrimingResult::eHSTS_PRIMING_FAILED_ACCEPT);
+                HSTSPrimingResult::eHSTS_PRIMING_FAILED_ACCEPT);
     }
 
     // Don't visit again for at least
     // security.mixed_content.hsts_priming_cache_timeout seconds.
     nsISiteSecurityService* sss = gHttpHandler->GetSSService();
     NS_ENSURE_TRUE(sss, NS_ERROR_OUT_OF_MEMORY);
     nsresult rv = sss->CacheNegativeHSTSResult(mURI,
             nsMixedContentBlocker::sHSTSPrimingCacheTimeout);

--- a/netwerk/protocol/http/nsIHstsPrimingCallback.idl
+++ b/netwerk/protocol/http/nsIHstsPrimingCallback.idl
@@ -28,16 +28,17 @@ interface nsIHstsPrimingCallback : nsISu
    *
    * May be invoked synchronously if HSTS priming has already been performed
    * for the host.
    *
    * @param aCached whether the result was already in the HSTS cache
    */
   [noscript, nostdcall]
   void onHSTSPrimingSucceeded(in bool aCached);
+
   /**
    * HSTS priming has seen no STS header, the request itself has failed,
    * or some other failure which does not constitute a positive signal that the
    * site can be upgraded safely to HTTPS. The request may still be allowed
    * based on the user's preferences.
    *
    * May be invoked synchronously if HSTS priming has already been performed
    * for the host.

--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -8143,17 +8143,17 @@
     "description": "How often would blocked mixed content be allowed if HSTS upgrades were allowed, including how often would we send an HSTS priming request? 0=display/no-HSTS, 1=display/HSTS, 2=active/no-HSTS, 3=active/HSTS, 4=display/no-HSTS-priming, 5=display/do-HSTS-priming, 6=active/no-HSTS-priming, 7=active/do-HSTS-priming"
   },
   "MIXED_CONTENT_HSTS_PRIMING_RESULT": {
     "alert_emails": ["seceng@mozilla.org"],
     "bug_numbers": [1246540],
     "expires_in_version": "60",
     "kind": "enumerated",
     "n_values": 16,
-    "description": "How often do we get back an HSTS priming result which upgrades the connection to HTTPS? 0=cached (no upgrade), 1=cached (do upgrade), 2=cached (blocked), 3=already upgraded, 4=priming succeeded, 5=priming succeeded (block due to pref), 6=priming succeeded (no upgrade due to pref), 7=priming failed (block), 8=priming failed (accept)"
+    "description": "How often do we get back an HSTS priming result which upgrades the connection to HTTPS? 0=cached (no upgrade), 1=cached (do upgrade), 2=cached (blocked), 3=already upgraded, 4=priming succeeded, 5=priming succeeded (block due to pref), 6=priming succeeded (no upgrade due to pref), 7=priming failed (block), 8=priming failed (accept), 9=timeout (block), 10=timeout (accept)"
   },
   "HSTS_PRIMING_REQUEST_DURATION": {
     "alert_emails": ["seceng-telemetry@mozilla.org"],
     "bug_numbers": [1311893],
     "expires_in_version": "58",
     "kind": "exponential",
     "low": 100,
     "high": 30000,

--- a/xpcom/base/ErrorList.h
+++ b/xpcom/base/ErrorList.h
@@ -333,16 +333,20 @@
   ERROR(NS_NET_STATUS_CONNECTED_TO,    FAILURE(4)),
   ERROR(NS_NET_STATUS_SENDING_TO,      FAILURE(5)),
   ERROR(NS_NET_STATUS_WAITING_FOR,     FAILURE(10)),
   ERROR(NS_NET_STATUS_RECEIVING_FROM,  FAILURE(6)),
 
   /* nsIInterceptedChannel */
   /* Generic error for non-specific failures during service worker interception */
   ERROR(NS_ERROR_INTERCEPTION_FAILED,                  FAILURE(100)),
+
+  /* nsIHstsPrimingListener */
+  /* Error code for HSTS priming timeout to distinguish from blocking */
+  ERROR(NS_ERROR_HSTS_PRIMING_TIMEOUT, FAILURE(110)),
 #undef MODULE
 
 
   /* ======================================================================= */
   /* 7: NS_ERROR_MODULE_PLUGINS */
   /* ======================================================================= */
 #define MODULE NS_ERROR_MODULE_PLUGINS
   ERROR(NS_ERROR_PLUGINS_PLUGINSNOTCHANGED,        FAILURE(1000)),