diff --git a/mozilla/js/macbuild/JavaScript.Prefix b/mozilla/js/macbuild/JavaScript.Prefix
index 19ed3abadcf..652b060b25a 100644
--- a/mozilla/js/macbuild/JavaScript.Prefix
+++ b/mozilla/js/macbuild/JavaScript.Prefix
@@ -29,3 +29,8 @@
 
 #include "MacPrefix.h"
 #include "JavaScriptConfig.h"
+
+// Bump down the optimization level to avoid a bug in the Pro 7 optimizer
+// with JS_ISIDENT() called with -1 (see bug 114915).
+
+#pragma optimization_level     2