With the suspended Jurgen Klopp watching on from the stands, Liverpool were not at the races in the opening exchanges and deservedly went behind to Jacob Ramsey's strike, but Firmino signed off at Anfield with an 89th-minute equaliser - his final goal in front of the home faithful, and potentially a priceless one as the Reds remain mathematically capable of securing a top-four finish.
As the news filtered through that top-four rivals Manchester United had taken an early lead against Bournemouth, the mood around Anfield was hardly a jubilant one, and the groans grew louder when Ibrahima Konate clumsily brought down Ollie Watkins inside the area with 19 minutes gone.
John Brooks had no choice but to point to the spot - booking Konate in the process - and there could have been few complaints from those in red, but they were handed an early-let off as Watkins went for placement from 12 yards and curled his penalty wide.
Liverpool did not accept that gift, though, as Aston Villa continued to dominate proceedings against a strangely lacklustre Reds side, who would eventually be punished for their passive start only six minutes after Watkins fluffed his lines.
Following a failed corner, Villa continued to knock the ball around the Liverpool penalty area, and Douglas Luiz was allowed time and space to send a cross into the back post, where Ramsey directed a smart volley back across goal and into the side of the net from a tight angle.
Ramsey could have easily doubled his tally in the 38th minute, but Alisson Becker was equal to his angled effort this time around, before Luis Diaz sent a header wide from Jordan Henderson's cross seconds later following a rapid Reds counter-attack.
Besides that half-chance for Diaz, Liverpool were largely toothless, but Villa were the beneficiaries of two controversial officiating calls towards the end of the half, as Tyrone Mings firstly caught Cody Gakpo in the midriff with his studs, but a yellow card was deemed sufficient punishment.
Halfway through a nine-minute added time period, Henderson went down in the box under pressure from Luiz, but the officials - who may have been thankful that Klopp was confined to the stands - waved away the captain's appeals.
For the first time under Klopp at Anfield, Liverpool failed to have a single shot on target in the first half, but the Reds thought that they had restored parity early in the second 45, as Gakpo poked home from close range following an almighty goalmouth scramble.
However, Brooks was peculiarly sent to look at the pitchside monitor to review a possible offside, and following a lengthy delay, the goal was eventually chalked off; Virgil van Dijk was deemed guilty of the offence.
The ball had flicked off Ezri Konsa before making its way to Van Dijk, but the Villa defender's action was deemed unintentional, and Liverpool continued to struggle to build momentum as Unai Emery's players began dropping to the turf all over the shop.
Firmino and James Milner were brought on for their Anfield swansongs in the final 20 minutes as Klopp threw men forward, and of course, the script was written for the former to be the Reds' saviour.
With 89 minutes gone, a neat outside-of-the-foot cross from Mohamed Salah found the run of Firmino, who stuck out a boot and poked home a close-range volley in front of the Kop.
The hosts were granted 10 minutes of added time to try to find a winner, and they handed Villa several scares as the Lions adopted a backs-against-the-wall approach, but Klopp's men would be forced to accept just the one point.
With Manchester United edging past Bournemouth, defeat for Liverpool would have condemned the Reds to Europa League football, but Klopp's fifth-placed side are now three points behind the Red Devils with one game to play.
However, Erik ten Hag's side have a match in hand on their Merseyside adversaries, while Aston Villa have risen above Tottenham Hotspur into seventh place, level on points with Brighton & Hove Albion having played two games more.
Liverpool's final game of the season takes them to already-relegated Southampton next Sunday, while Emery's team host Brighton in a pivotal battle in the race for European football.