Brentford's Ivan Toney got his 30th goal of the season to equal the Championship record and help his side secure a third-placed finish as they beat second-placed Watford.
But they had to wait just 53 seconds after the restart to go ahead as Forss slotted in from close range at the far post after Sergi Canos had got to the byeline.
Having been fouled in the box by Francisco Sierralta, Toney calmly slotted home the penalty after an hour to draw level with Glenn Murray's record for Crystal Palace with a game to go.
The victory ensured a second successive third-placed finish for Brentford, who lost to Fulham in last season's play-off final.