FSM actors cannot be modified with stackable traits like

trait MyStackableTrait extends Actor {
  abstract override def receive: Receive = {
    case msg => {
      // do some extra work
      // ...
      super.receive(msg)
    }
  }
}

because FSM.receive is implemented as final method. Making FSM.receive overridable (i.e. non-final) would also open up FSM actors for composition with stackable traits. See also https://groups.google.com/d/msg/akka-user/3ySTQvS0vFE/4CmCB7dct9MJ.