With Liverpool leading 1-0 through a stunning Philippe Coutinho goal, Firmino wasted a glorious opportunity to double the hosts’ lead.
The Brazilian had the goal at his mercy but instead switched the ball onto his favoured right foot and dragged his effort wide of Lukasz Fabianski’s goal.
Redknapp says the likes of Tottenham star Harry Kane and Manchester City forward Sergio Aguero would have scored, and believes it shows a lack of ruthlessness from Liverpool.
“This is game over. I think if Liverpool go in 2-0, I don’t see a way back for Swansea,” Redknapp said on Sky Sports.
“Again they lose it in their own half, this is the chance, finish. You’ve got to finish that.
“He tries to be a bit clever, he could lay it to someone else, and he knows it.
“That’s the game won and I’m sure Jurgen Klopp would have thought to himself ‘I can relax, we can maybe bring on some substitutes’.
“Absolutely (it’s Liverpool Achilles heel).
“You look at Harry Kane for instance, Aguero, they’re finishing there. That’s the difference I think.
“Their football is fantastic, but they get in so many great positions to finish off teams, to punish them and they just don’t have that in their locker really.
“That’s the problem and that’s what’s going to cost them.”
Firmino made up for his miss, doubling Liverpool’s lead just after half time.
He added another before Trent Alexander-Arnold and Alex Oxlade-Chamberlain sealed a 5-0 win for the Reds.