--- a/toolkit/components/url-classifier/nsUrlClassifierStreamUpdater.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierStreamUpdater.cpp
@@ -26,16 +26,20 @@
 #include "nsIURLFormatter.h"
 #include "Classifier.h"
 
 static const char* gQuitApplicationMessage = "quit-application";
 
 // Limit the list file size to 32mb
 const uint32_t MAX_FILE_SIZE = (32 * 1024 * 1024);
 
+// Retry delay when we failed to DownloadUpdate() if due to
+// DBService busy.
+const uint32_t FETCH_NEXT_REQUEST_RETRY_DELAY_MS = 1000;
+
 #undef LOG
 
 // MOZ_LOG=UrlClassifierStreamUpdater:5
 static mozilla::LazyLogModule gUrlClassifierStreamUpdaterLog("UrlClassifierStreamUpdater");
 #define LOG(args) TrimAndLog args
 
 // Calls nsIURLFormatter::TrimSensitiveURLs to remove sensitive
 // info from the logging message.
@@ -277,16 +281,27 @@ nsUrlClassifierStreamUpdater::DownloadUp
     PendingRequest *request = mPendingRequests.AppendElement();
     request->mTables = aRequestTables;
     request->mRequestPayload = aRequestPayload;
     request->mIsPostRequest = aIsPostRequest;
     request->mUrl = aUpdateUrl;
     request->mSuccessCallback = aSuccessCallback;
     request->mUpdateErrorCallback = aUpdateErrorCallback;
     request->mDownloadErrorCallback = aDownloadErrorCallback;
+
+    // We cannot guarantee that we will be notified when DBService is done
+    // processing the current update, so we fire a retry timer on our own.
+    nsresult rv;
+    mFetchNextRequestTimer = do_CreateInstance("@mozilla.org/timer;1", &rv);
+    if (NS_SUCCEEDED(rv)) {
+      rv = mFetchNextRequestTimer->InitWithCallback(this,
+                                                    FETCH_NEXT_REQUEST_RETRY_DELAY_MS,
+                                                    nsITimer::TYPE_ONE_SHOT);
+    }
+
     return NS_OK;
   }
 
   if (NS_FAILED(rv)) {
     return rv;
   }
 
   nsCOMPtr<nsIUrlClassifierUtils> urlUtil =
@@ -409,20 +424,20 @@ nsUrlClassifierStreamUpdater::StreamFini
     return NS_OK;
   }
 
   // This timer is for fetching indirect updates ("forwards") from any "u:" lines
   // that we encountered while processing the server response. It is NOT for
   // scheduling the next time we pull the list from the server. That's a different
   // timer in listmanager.js (see bug 1110891).
   nsresult rv;
-  mTimer = do_CreateInstance("@mozilla.org/timer;1", &rv);
+  mFetchIndirectUpdatesTimer = do_CreateInstance("@mozilla.org/timer;1", &rv);
   if (NS_SUCCEEDED(rv)) {
-    rv = mTimer->InitWithCallback(this, requestedDelay,
-                                  nsITimer::TYPE_ONE_SHOT);
+    rv = mFetchIndirectUpdatesTimer->InitWithCallback(this, requestedDelay,
+                                                      nsITimer::TYPE_ONE_SHOT);
   }
 
   if (NS_FAILED(rv)) {
     NS_WARNING("Unable to initialize timer, fetching next safebrowsing item immediately");
     return FetchNext();
   }
 
   return NS_OK;
@@ -812,19 +827,23 @@ nsUrlClassifierStreamUpdater::Observe(ns
       LOG(("Cancel download"));
       nsresult rv;
       rv = mChannel->Cancel(NS_ERROR_ABORT);
       NS_ENSURE_SUCCESS(rv, rv);
       mIsUpdating = false;
       mChannel = nullptr;
       mTelemetryClockStart = 0;
     }
-    if (mTimer) {
-      mTimer->Cancel();
-      mTimer = nullptr;
+    if (mFetchIndirectUpdatesTimer) {
+      mFetchIndirectUpdatesTimer->Cancel();
+      mFetchIndirectUpdatesTimer = nullptr;
+    }
+    if (mFetchNextRequestTimer) {
+      mFetchNextRequestTimer->Cancel();
+      mFetchNextRequestTimer = nullptr;
     }
   }
   return NS_OK;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 // nsIInterfaceRequestor implementation
 
@@ -837,16 +856,25 @@ nsUrlClassifierStreamUpdater::GetInterfa
 
 ///////////////////////////////////////////////////////////////////////////////
 // nsITimerCallback implementation
 NS_IMETHODIMP
 nsUrlClassifierStreamUpdater::Notify(nsITimer *timer)
 {
   LOG(("nsUrlClassifierStreamUpdater::Notify [%p]", this));
 
-  mTimer = nullptr;
+  if (timer == mFetchNextRequestTimer) {
+    mFetchNextRequestTimer = nullptr;
+    FetchNextRequest();
+    return NS_OK;
+  }
 
-  // Start the update process up again.
-  FetchNext();
+  if (timer == mFetchIndirectUpdatesTimer) {
+    mFetchIndirectUpdatesTimer = nullptr;
+    // Start the update process up again.
+    FetchNext();
+    return NS_OK;
+  }
 
+  MOZ_ASSERT_UNREACHABLE("A timer is fired from nowhere.");
   return NS_OK;
 }
 

--- a/toolkit/components/url-classifier/nsUrlClassifierStreamUpdater.h
+++ b/toolkit/components/url-classifier/nsUrlClassifierStreamUpdater.h
@@ -76,17 +76,25 @@ private:
 
   nsCString mDownloadErrorStatusStr;
 
   // Note that mStreamTable is only used by v2, it is empty for v4 update.
   nsCString mStreamTable;
 
   nsCOMPtr<nsIChannel> mChannel;
   nsCOMPtr<nsIUrlClassifierDBService> mDBService;
-  nsCOMPtr<nsITimer> mTimer;
+
+  // In v2, a update response might contain redirection and this
+  // timer is for fetching the redirected update.
+  nsCOMPtr<nsITimer> mFetchIndirectUpdatesTimer;
+
+  // When we DownloadUpdate(), the DBService might be busy on processing
+  // request issused outside of StreamUpdater. We have to fire a timer to
+  // retry on our own.
+  nsCOMPtr<nsITimer> mFetchNextRequestTimer;
 
   struct PendingRequest {
     nsCString mTables;
     nsCString mRequestPayload;
     bool mIsPostRequest;
     nsCString mUrl;
     nsCOMPtr<nsIUrlClassifierCallback> mSuccessCallback;
     nsCOMPtr<nsIUrlClassifierCallback> mUpdateErrorCallback;