Put Coutinho next to the right person in a midfield two and it could work. Arsenal have dropped Carzola into that midfield two position with good effect this season and I could see Coutinho having a similar influence.
As for Firmino, he's still consistently inconsistent. He has a habit of drifting in and out of games. Still not sure what his best position is.
Firmino's best position is quite clearly up front, with a licence to roam. He is inconsistent in the last few games because, quite frankly, him and Sturridge don't know what each other is doing yet. We need to let them play together consistently to get some understanding. Neither of them are adapting their game for each other so its a case of letting them play and then letting them understand what each other wants to do.
I think Firmino would do better with a direct striker like Torres, Origi, Owen etc. in the short term whilst Sturridge would do better with someone who will work the defenders, play the channels etc. Because they both want the ball to feet its all a bit samey.
I don't agree about Coutinho. If we play him in the centre then thats Firmino, Sturridge, Coutinho clogging up the centre. All 3 like to drop into a similar area.