diff --git a/xpcom/typelib/xpt/public/xpt_struct.h b/xpcom/typelib/xpt/public/xpt_struct.h
index e4eaaa6af720..f71e90e6c86b 100644
--- a/xpcom/typelib/xpt/public/xpt_struct.h
+++ b/xpcom/typelib/xpt/public/xpt_struct.h
@@ -206,11 +206,16 @@ XPT_DestroyInterfaceDirectoryEntry(XPTArena *arena,
  * single XPCOM interface, including all of its methods. 
  */
 struct XPTInterfaceDescriptor {
+    /* This field ordering minimizes the size of this struct.
+    *  The fields are serialized on disk in a different order.
+    *  See DoInterfaceDescriptor().
+    */
+    XPTMethodDescriptor     *method_descriptors;
+    XPTConstDescriptor      *const_descriptors;
+    XPTTypeDescriptor       *additional_types;
     uint16_t                parent_interface;
     uint16_t                num_methods;
-    XPTMethodDescriptor     *method_descriptors;
     uint16_t                num_constants;
-    XPTConstDescriptor      *const_descriptors;
     uint8_t                 flags;
 
     /* additional_types are used for arrays where we may need multiple
@@ -229,7 +234,6 @@ struct XPTInterfaceDescriptor {
     *  for that.
     */
 
-    XPTTypeDescriptor       *additional_types;
     uint16_t                num_additional_types;
 };