--- a/dom/quota/ActorsParent.cpp
+++ b/dom/quota/ActorsParent.cpp
@@ -4432,17 +4432,17 @@ QuotaManager::EnsureOriginIsInitialized(
     if (NS_WARN_IF(NS_FAILED(rv))) {
       // We have to cleanup partially initialized quota.
       RemoveQuota();
 
       return rv;
     }
 
     if (gFixedLimitKB >= 0) {
-      mTemporaryStorageLimit = gFixedLimitKB * 1024;
+      mTemporaryStorageLimit = static_cast<uint64_t>(gFixedLimitKB) * 1024;
     }
     else {
       nsCOMPtr<nsIFile> storageDir =
         do_CreateInstance(NS_LOCAL_FILE_CONTRACTID, &rv);
       if (NS_WARN_IF(NS_FAILED(rv))) {
         return rv;
       }