The Brazilian, who started in a deeper role for the Reds against Neil Warnock's side, was pinned back against his own touch-line whilst tracking back.
Firmino looked to have conceded the ball or given away a corner as he fell to the ground against the on-rushing Víctor Camarasa.
However, he pulled off a crazy bit of skill, nutmegging the Spaniard with a no-look back-heel.
The Reds currently lead 1-0 at the break having dominated possession, with Mohamed Salah scoring his third in two games.
Watch Firmino's excellent skill in all its glory below...
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FIRMINO JUST DID A BACKHEEL NUTMEG ON THE FLOOR I THINK?!
— Empire of the Kop (@empireofthekop) October 27, 2018
As if he’s just done a backheel nutmeg while on the floor. Firmino is incredible.
— Leanne Prescott (@_lfcleanne) October 27, 2018
Firmino just did a backheel nutmeg on the floor, magic
— - (@AllezTheReds) October 27, 2018