diff --git a/js/src/config.mk b/js/src/config.mk
index f622d30a81b6..fb7e13d0a649 100644
--- a/js/src/config.mk
+++ b/js/src/config.mk
@@ -117,7 +117,11 @@ CP = cp
 endif
 
 ifdef BUILD_OPT
-OPTIMIZER  = -O
+ifdef USE_MSVC
+OPTIMIZER  = -O1
+else
+OPTIMIZER  = -Os
+endif
 DEFINES    += -UDEBUG -DNDEBUG -UDEBUG_$(USER)
 OBJDIR_TAG = _OPT
 else