Tony Pulis’ side have inconvenienced other Champions League aspirants of late, beating Arsenal 3-1 and holding Manchester United to a 0-0 draw.
But they were unable to lay a glove on Liverpool for much of a listless encounter, and what is normally an area of strength for Albion proved to be their downfall.
Firmino was allowed too much room to net his 11th Premier League goal of the season as the hosts’ set-piece defending failed them in first-half stoppage time.
Jurgen Klopp’s side provided few of the thrills normally associated with the division’s top scorers, with Sadio Mane’s void in attack one they will struggle to fill over the closing weeks, but Liverpool never looked like letting their control slip during the second period.
They return to third place, two points above Manchester City having played a game more.