I don't call goal production statistic really
You try to act like Firmino is playing CB really, it's laughable.
Firmino is in the box of every Liverpool attack I'm looking. Just because he helps in the buildup doesn't mean he's not in the box. And Giroud does help in the buildup too.
So you aren't watching then, because this simply isn't true. When a player is in the box and where makes a big difference as well. I'll give some examples of their heat maps:
Firmino first with a season heatmap in the season he spent the most time forward (17/18):
Here's another of Firmino:
Here are a couple from Giroud:
Now, again, Giroud has some games where he's a bit more withdrawn but most of his high scoring games or games that he scores in look like that. There's a massive difference between Firmino as a 'striker' and most strikers. He is essentially a 10 that starts higher to lead the press but in possession plays more to the natural 10 position and roams around a lot. He does get into the box but nowhere near as much as traditional strikers do, and he's never the player that they're looking to get the ball to to score. In fact, more often than not, when he gets the ball in the box he's looking to move it on to someone else and create a better scoring opportunity.