Firmino struck the second of Liverpool's four goals at St Mary's Stadium, with Diogo Jota finding the back of the net twice, alongside a solitary effort from substitute Cody Gakpo.
Jürgen Klopp's men went ahead initially in the contest, before scoring twice in two minutes in the second period to secure a point on the south coast on the final day of the 2022-23 campaign.
Team news
Liverpool's final teamsheet of the season displayed seven changes – including starts for the departing Firmino and James Milner.
Elsewhere, Caoimhin Kelleher was named in goal and Joe Gomez, Joel Matip and Kostas Tsimikas were selected in defence, while Jota returned to the attack.
Liverpool: Kelleher, Alexander-Arnold, Matip, Gomez, Tsimikas (Elliott, 56), Fabinho (Henderson, 57), Milner, Jones (Gakpo, 56), Jota, Salah, Firmino (Diaz, 57).
Unused subs: Adrian, Konate, Carvalho, Arthur, Williams.
First half
The game began in quiet fashion though burst into life on 10 minutes as the Reds pressed the home side high up the pitch.
Jota swept in on the defence to lead the charge and stole a loose ball, before firing into an empty net to open the scoring in clinical fashion.
Just three minutes later Firmino – like he has done so many times before – danced into the box and calmly drilled a low effort into the goal to send the visiting crowd wild.
Home captain James Ward-Prowse then struck one back amid a flurry of action in the opening 20 minutes of the game.
The midfielder found space inside the area to sweep into the bottom corner of the goal and reduce the deficit to just one.
Matip and Jota both threatened to add to the scoreline with headers but it was the hosts who levelled the game through Kamaldeen Sulemana on the half-hour mark.
Saints broke and the forward finished well to ensure the two teams entered the interval all square.
Second half
The home team completed the turnaround three minutes into the second half with Sulemana adding his second of the afternoon.
The forward burst through into the Reds half to find the bottom corner of the goal and give his team their first advantage of the day.
Liverpool responded, as Mohamed Salah proved a threat down the right flank, seeing a number of shots blocked on separate occasions.
Southampton, though, added a fourth strike a short while later as Adam Armstrong curled a low shot into the net during a decisive breakaway.
Liverpool were in no mood to lie down, and they struck twice in two minutes to once again level a topsy-turvy tie on the south coast.
Substitute Gakpo first tapped in from close range at the back post from Trent Alexander-Arnold's low cross, before Salah played in Jota for a thunderous strike from the restart to earn a fourth.
Salah then almost put Liverpool ahead, hitting the post with a dinked volley from Jordan Henderson's pass over the top of the Saints defence.
Gakpo saw another effort fly over the bar in the latter stages, while Henderson's volley from distance was saved well by Alex McCarthy.
The home goalkeeper was again forced into late action as Salah's shot was beaten away to ensure the fixture concluded with a point apiece.