Mohamed Salah scored another outstanding individual goal as Claudio Ranieri endured a nightmare start to life at Watford.
Salah danced past three Watford defenders, left Craig Cathcart on his backside and then curled a wonderful effort into the far corner to make it 4-0 in the second half.
Sadio Mane had earlier put Liverpool ahead after just nine minutes with his 100th Premier League goal following a subline pass from Salah.
Firmino doubled the lead on 37 minutes and the Brazilian got his second on 52 minutes when he tapped home from close range.
But Salah stole the show with another brilliant goal to show why many people believe he is the best player in the world right now. When he is playing like this, it is hard to disagree.
Firmino completed his hat-trick in stoppage time.
Watford were woeful and if Ranieri did not already know he has a job on his hands then he does now.
The win moves Liverpool top of the Premier League, with Chelsea playing Brentford later.
