Firmino's natural position was number 10 before he started playing striker for us. Look at his position you will see he spends a lot of time closer to the centre with both Salah and Mane ahead of him. Coutinho hasn't excelled in the middle, his best position has been on the left, Klopp has been trying to convert him in a more central position since the arrival of Salah who moved Mane on the left. You can laugh if you want, doesn't mean you have exclusivity on the truth.
I just understand the way we set up and the way we play, which isn't with a number 10. There is nothing to suggest that Firmino could be one of the central midfield 3 and there is no reason to move him back there when his best position is clearly a striker.
The reason he plays deeper is to link the play and use the pace of Salah and Mane further forward.