--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5183,16 +5183,18 @@ pref("browser.safebrowsing.allowOverride
 #ifdef MOZILLA_OFFICIAL
 // Normally the "client ID" sent in updates is appinfo.name, but for
 // official Firefox releases from Mozilla we use a special identifier.
 pref("browser.safebrowsing.id", "navclient-auto-ffox");
 #else
 pref("browser.safebrowsing.id", "Firefox");
 #endif
 
+pref("browser.safebrowsing.temporary.take_v4_completion_result", false);
+
 // Turn off Spatial navigation by default.
 pref("snav.enabled", false);
 
 // Debug-only pref to force enable the AccessibleCaret. If you want to
 // control AccessibleCaret by mouse, you'll need to set
 // "layout.accessiblecaret.hide_carets_for_mouse_input" to false.
 pref("layout.accessiblecaret.enabled", false);
 

--- a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
@@ -117,16 +117,21 @@ LazyLogModule gUrlClassifierDbServiceLog
 #define BLOCKED_TABLE_PREF              "urlclassifier.blockedTable"
 #define DOWNLOAD_BLOCK_TABLE_PREF       "urlclassifier.downloadBlockTable"
 #define DOWNLOAD_ALLOW_TABLE_PREF       "urlclassifier.downloadAllowTable"
 #define DISALLOW_COMPLETION_TABLE_PREF  "urlclassifier.disallow_completions"
 
 #define CONFIRM_AGE_PREF        "urlclassifier.max-complete-age"
 #define CONFIRM_AGE_DEFAULT_SEC (45 * 60)
 
+// TODO: The following two prefs are to be removed after we
+//       roll out full v4 hash completion. See Bug 1331534.
+#define TAKE_V4_COMPLETION_RESULT_PREF    "browser.safebrowsing.temporary.take_v4_completion_result"
+#define TAKE_V4_COMPLETION_RESULT_DEFAULT false
+
 class nsUrlClassifierDBServiceWorker;
 
 // Singleton instance.
 static nsUrlClassifierDBService* sUrlClassifierDBService;
 
 nsIThread* nsUrlClassifierDBService::gDbBackgroundThread = nullptr;
 
 // Once we've committed to shutting down, don't do work in the background
@@ -1050,16 +1055,25 @@ nsUrlClassifierLookupCallback::Completio
 
 NS_IMETHODIMP
 nsUrlClassifierLookupCallback::Completion(const nsACString& completeHash,
                                           const nsACString& tableName,
                                           uint32_t chunkId)
 {
   LOG(("nsUrlClassifierLookupCallback::Completion [%p, %s, %d]",
        this, PromiseFlatCString(tableName).get(), chunkId));
+
+  if (StringEndsWith(tableName, NS_LITERAL_CSTRING("-proto")) &&
+      !Preferences::GetBool(TAKE_V4_COMPLETION_RESULT_PREF,
+                            TAKE_V4_COMPLETION_RESULT_DEFAULT)) {
+    // Bug 1331534 - We temporarily ignore hash completion result
+    // for v4 tables.
+    return NS_OK;
+  }
+
   mozilla::safebrowsing::Completion hash;
   hash.Assign(completeHash);
 
   // Send this completion to the store for caching.
   if (!mCacheResults) {
     mCacheResults = new CacheResultArray();
     if (!mCacheResults)
       return NS_ERROR_OUT_OF_MEMORY;