Any player is underrated, if you ignore everyone else praising them. Firmino's been getting a lot of recognition from the pundits and journalists all season.
Fair enough, I don't really read the English press or get to see many English football shows.
In my realm, he is very much underrated. Even among many Liverpool fans I have to talk him up. Pundits share some praise here and there, but mostly after doing something splendid like yesterday. Most fail to recognise his importance to our team, what he brings away from the pretty things. Some even say, still, that he needs to add goals to be recognised as a really good player.
So I guess it depends on where you are to some extent.