The Brazilian was handed a start by Jurgen Klopp in what will be his last ever appearance for the club.
Diogo Jota opened the scoring against Southampton before the Brazilian netted a second.
He nutmegged Saints goalkeeper Alex McCarthy at the St Mary's in the 14th minute.
The goals came thick and fast, however, as James Ward-Prowse got one back just five minutes later.
And then Saints completed the comeback with Kamaldeen Sulemana making it 2-2, firing home low with the departing Theo Walcott grabbing the assist.
Read more on Premier League
Xhaka scores twice in potential farewell Arsenal game as fans urge club to keep midfielder
Kane equals Premier League record and hands Leeds worst possible start in survival bid
Firmino's next club has not yet been confirmed, but both Real Madrid and Barcelona have been linked.
The Brazilian is not the only player making his last appearance for Liverpool on Sunday.
James Milner was also named in the starting line-up by Klopp, with the veteran midfielder likely to sign for Brighton.
Goalkeeper Caoimhin Kelleher, who has also been linked with a move away this summer, was also named in the starting XI in place of Alisson Becker.