Coutinho and Firmino played alongside one another at Liverpool for two-and-a-half years, and struck up a friendship in that time.
In fact, the new Barcelona star was best man at Firmino's wedding to Larissa Pereira in Brazil last summer.
On Facebook, the Reds attacker wrote: "Liverpool is no longer the same without you my magical brother. I wish you infinite success, that you enjoy life and realise all your dreams brother. God is always protecting you and your whole family. It was an honour to play with a magician like you."
Coutinho spent five years at Liverpool but did not win a trophy in that time.