diff --git a/mfbt/DebugOnly.h b/mfbt/DebugOnly.h
index 451d9bc3738..3190107958c 100644
--- a/mfbt/DebugOnly.h
+++ b/mfbt/DebugOnly.h
@@ -30,8 +30,9 @@ namespace mozilla {
  * DebugOnly instances can only be coerced to T in debug builds.  In release
  * builds they don't have a value, so type coercion is not well defined.
  *
- * Note that DebugOnly instances still take up one byte of space, plus padding,
- * when used as members of structs.
+ * NOTE! DebugOnly instances still take up one byte of space, plus padding, even
+ * in optimized, non-DEBUG builds.  Don't use DebugOnly for struct/class members
+ * unless that really doesn't matter to you.
  */
 template<typename T>
 class DebugOnly