The implication is that sales have gone down also. If that is **not** the case, then hiring an outside marketing firm can be a good Idea and then you can re-position your marketing team to do other things.
If sales have plunged as a result of the stagnation of the marketing department, then It would be more effective to replace the marketing manager. Everything rises and falls on leadership, and if the marketing manager is slacking, that attitude will permeate the selection and working relationship with an outside firm.
Logically if the marketing manager cannot motivate himself or his team how can they manage an outside team. (Also a plunge in sales would mean lack of resources to hire outside firms)
An outside firm should be reserved for those that are doing well, and need to move personnel from marketing to customer service to facilitate the influx of new business that was created from their efforts in the first place.