--- a/memory/jemalloc/jemalloc.c
+++ b/memory/jemalloc/jemalloc.c
@@ -100,16 +100,23 @@
  * defaults the A and J runtime options to off.  These settings are appropriate
  * for production systems.
  */
 #ifndef MOZ_MEMORY_DEBUG
 #  define	MALLOC_PRODUCTION
 #endif
 
 /*
+ * Use only one arena by default.  Mozilla does not currently make extensive
+ * use of concurrent allocation, so the increased fragmentation associated with
+ * multiple arenas is not warranted.
+ */
+#define	MOZ_MEMORY_NARENAS_DEFAULT_ONE
+
+/*
  * MALLOC_STATS enables statistics calculation, and is required for
  * jemalloc_stats().
  */
 #define MALLOC_STATS
 
 #ifndef MALLOC_PRODUCTION
    /*
     * MALLOC_DEBUG enables assertions and other sanity checks, and disables
@@ -1043,27 +1050,17 @@ static __thread arena_t	*arenas_map;
 /* Chunk statistics. */
 static chunk_stats_t	stats_chunks;
 #endif
 
 /*******************************/
 /*
  * Runtime configuration options.
  */
-const char	*_malloc_options
-#ifdef MOZ_MEMORY_WINDOWS
-= "A10n"
-#elif (defined(MOZ_MEMORY_DARWIN))
-= "AP10n"
-#elif (defined(MOZ_MEMORY_LINUX))
-= "A10n"
-#elif (defined(MOZ_MEMORY_SOLARIS))
-= "A10n"
-#endif
-;
+const char	*_malloc_options;
 
 #ifndef MALLOC_PRODUCTION
 static bool	opt_abort = true;
 #ifdef MALLOC_FILL
 static bool	opt_junk = true;
 #endif
 #else
 static bool	opt_abort = false;
@@ -5620,26 +5617,30 @@ MALLOC_OUT:
 	 * the use of space that would otherwise be wasted.
 	 */
 	if (opt_dss)
 		base_pages_alloc(0);
 #endif
 	base_nodes = NULL;
 	malloc_mutex_init(&base_mtx);
 
+#ifdef MOZ_MEMORY_NARENAS_DEFAULT_ONE
+	narenas = 1;
+#else
 	if (ncpus > 1) {
 		/*
 		 * For SMP systems, create four times as many arenas as there
 		 * are CPUs by default.
 		 */
 		opt_narenas_lshift += 2;
 	}
 
 	/* Determine how many arenas to use. */
 	narenas = ncpus;
+#endif
 	if (opt_narenas_lshift > 0) {
 		if ((narenas << opt_narenas_lshift) > narenas)
 			narenas <<= opt_narenas_lshift;
 		/*
 		 * Make sure not to exceed the limits of what base_alloc() can
 		 * handle.
 		 */
 		if (narenas * sizeof(arena_t *) > chunksize)