--- a/toolkit/components/url-classifier/HashStore.cpp
+++ b/toolkit/components/url-classifier/HashStore.cpp
@@ -37,16 +37,17 @@
 #include "nsCheckSummedOutputStream.h"
 #include "prio.h"
 #include "mozilla/Logging.h"
 #include "mozilla/IntegerPrintfMacros.h"
 #include "mozilla/SizePrintfMacros.h"
 #include "zlib.h"
 #include "Classifier.h"
 #include "nsUrlClassifierDBService.h"
+#include "nsDirectoryServiceDefs.h"
 #include "mozilla/Telemetry.h"
 
 // Main store for SafeBrowsing protocol data. We store
 // known add/sub chunks, prefixes and completions in memory
 // during an update, and serialize to disk.
 // We do not store the add prefixes, those are retrieved by
 // decompressing the PrefixSet cache whenever we need to apply
 // an update.
@@ -306,27 +307,49 @@ HashStore::CheckChecksum(uint32_t aFileS
 
 nsresult
 HashStore::Open()
 {
   nsCOMPtr<nsIFile> storeFile;
   nsresult rv = mStoreDirectory->Clone(getter_AddRefs(storeFile));
   NS_ENSURE_SUCCESS(rv, rv);
 
-  rv = storeFile->AppendNative(mTableName + NS_LITERAL_CSTRING(".sbstore"));
+  rv = storeFile->AppendNative(mTableName + NS_LITERAL_CSTRING(STORE_SUFFIX));
   NS_ENSURE_SUCCESS(rv, rv);
 
   nsCOMPtr<nsIInputStream> origStream;
   rv = NS_NewLocalFileInputStream(getter_AddRefs(origStream), storeFile,
                                   PR_RDONLY | nsIFile::OS_READAHEAD);
 
   if (rv == NS_ERROR_FILE_NOT_FOUND) {
+    rv = NS_GetSpecialDirectory(NS_GRE_DIR,
+                                getter_AddRefs(storeFile));
+
+    rv = storeFile->AppendNative(NS_LITERAL_CSTRING("defaults"));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = storeFile->AppendNative(NS_LITERAL_CSTRING("safebrowsing"));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = storeFile->AppendNative(mTableName + NS_LITERAL_CSTRING(STORE_SUFFIX));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = NS_NewLocalFileInputStream(getter_AddRefs(origStream), storeFile,
+                                  PR_RDONLY | nsIFile::OS_READAHEAD);
+
+    if (rv != NS_ERROR_FILE_NOT_FOUND) {
+      LOG("Stored file was not found but a default file was found and will be used");
+    }
+  }
+
+  if (rv == NS_ERROR_FILE_NOT_FOUND) {
     UpdateHeader();
     return NS_OK;
   }
+
   NS_ENSURE_SUCCESS(rv, rv);
 
   int64_t fileSize;
   rv = storeFile->GetFileSize(&fileSize);
   NS_ENSURE_SUCCESS(rv, rv);
 
   if (fileSize < 0 || fileSize > UINT32_MAX) {
     return NS_ERROR_FAILURE;

--- a/toolkit/components/url-classifier/LookupCache.cpp
+++ b/toolkit/components/url-classifier/LookupCache.cpp
@@ -3,16 +3,17 @@
  * 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/. */
 
 #include "LookupCache.h"
 #include "HashStore.h"
 #include "nsISeekableStream.h"
 #include "mozilla/Telemetry.h"
 #include "mozilla/Logging.h"
+#include "nsDirectoryServiceDefs.h"
 #include "nsNetUtil.h"
 #include "prprf.h"
 #include "Classifier.h"
 
 // We act as the main entry point for all the real lookups,
 // so note that those are not done to the actual HashStore.
 // The latter solely exists to store the data needed to handle
 // the updates from the protocol.
@@ -406,16 +407,45 @@ LookupCache::LoadPrefixSet()
   if (exists) {
     LOG(("stored PrefixSet exists, loading from disk"));
     rv = LoadFromFile(psFile);
     if (NS_FAILED(rv)) {
       return rv;
     }
     mPrimed = true;
   } else {
+    nsCOMPtr<nsIFile> defaultPsFile;
+    rv = NS_GetSpecialDirectory(NS_GRE_DIR,
+                                getter_AddRefs(defaultPsFile));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = defaultPsFile->AppendNative(NS_LITERAL_CSTRING("defaults"));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = defaultPsFile->AppendNative(NS_LITERAL_CSTRING("safebrowsing"));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    rv = defaultPsFile->AppendNative(mTableName + NS_LITERAL_CSTRING(PREFIXSET_SUFFIX));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    bool defaultExists;
+    rv = defaultPsFile->Exists(&defaultExists);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    if (defaultExists) {
+      LOG(("default PrefixSet exists, loading from disk"));
+      rv = LoadFromFile(defaultPsFile);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      mPrimed = true;
+    }
+  }
+
+  if (!mPrimed) {
     LOG(("no (usable) stored PrefixSet found"));
   }
 
 #ifdef DEBUG
   if (mPrimed) {
     uint32_t size = SizeOfPrefixSet();
     LOG(("SB tree done, size = %d bytes\n", size));
   }