Tallying 46 in the Bonus Points System (BPS), the top score so far in Gameweek 33, the Brazilian fired a first-half winner and created five goalscoring opportunities.
Firmino also made 10 recoveries and two successful tackles, in addition to completing five dribbles.
Joel Matip (£5.3m) picked up two bonus points, with 33 in the BPS.
The Liverpool centre-back registered eight clearances, blocks and interceptions (CBI) and won three tackles to help earn his side a first clean sheet since Gameweek 25.
The Liverpool goalkeeper produced four CBI, 10 recoveries and two saves to deny the Baggies.
Lovren managed six CBI and three recoveries and also completed 87% of 77 attempted passes.
Firmino maintained his recent fine form to top the ICT Index with a score of 16.6.
His total of five key passes helped him to an ICT Creativity haul of 62, his biggest since Gameweek 6.
Firmino also registered eight touches in the opposition box and fired three shots on goal.
But it was his team-mate Philippe Coutinho (£8.5m) who boasted the main threat to the hosts in Sunday’s contest.
Subject to more than 89,000 transfers in for Gameweek 33 after scoring in three consecutive appearances, Coutinho produced four shots on goal.
Owned by 12.5% of FPL managers, Firmino has registered returns in each of his last six Gameweek appearances.
The Brazilian has three goals and four assists over that spell, with an 11-point haul on Sunday earning him a top-five place in the FPL midfield standings.
Liverpool’s lack of a Double Gameweek limits Firmino's appeal, however. Just over 9,000 FPL managers have acquired him in the Gameweek despite an otherwise favourable upcoming schedule.
According to the Fixture Difficulty Rankings (FDR), only one of Liverpool’s remaining five matches score more than two.
As the only side without a fixture in Gameweek 34, faith in West Bromwich Albion's regulars is in decline.
The 31.9%-owned Gareth McAuley (£5.3m) was sold by 44,000+ FPL managers for Gameweek 33 and has suffered a further 19,000 sales since the weekend, more than any defender.
McAuley has scored six times this term but has failed to register a single shot on goal in his last seven Gameweek appearances.
Elsewhere at the back, Ben Foster (£4.7m) is the second-most transferred-out goalkeeper ahead of Gameweek 34.
Tony Pulis’s side have mustered just a single clean sheet in their last nine Premier League outings.