diff --git a/build/automation.py.in b/build/automation.py.in
index 56e21c7f4472..e3b218a525a3 100644
--- a/build/automation.py.in
+++ b/build/automation.py.in
@@ -574,7 +574,11 @@ user_pref("camino.use_system_proxy_settings", false); // Camino-only, harmless t
     env['GNOME_DISABLE_CRASH_DIALOG'] = '1'
     env['XRE_NO_WINDOWS_CRASH_DIALOG'] = '1'
 
-    env['NS_TRACE_MALLOC_DISABLE_STACKS'] = '1'
+    # Don't do this for Mac since it makes the Mac OS X 10.5 (32-bit)
+    # trace-malloc leak test hang.  (It doesn't make the 10.6 (64-bit)
+    # leak test hang, though.)
+    if not self.IS_MAC:
+      env['NS_TRACE_MALLOC_DISABLE_STACKS'] = '1'
     return env
 
   if IS_WIN32: