Firmino falls into Def: 2 however I think its lazy analysis and scapegoating when Liverpool struggle in attack. He's much more like a #10 than a #9 but he leads the line.
@Lord Ron Saunders Messi is the original player for the position. He would draw the CBs out of their position leaving a vacuum behind him which Eto'o or Henry could sprint into.
The whole idea of it is the player in that position has to be dangerous enough to make the CBs feel like they to mark them or close them down. That breaks the defensive shape and create holes for players (wide players and midfielders) to run into. Grealish is actually a good example of a player who could do this role really well. Teams only recognize him as the danger. Kevin DeBruyne has played it in some games at City too.
They're not really the real threat but leaving them with lots of space is also too dangerous.