After agreeing a new deal to keep him under contract at the Wirsol Rhein-Neckar-Arena until 2017, Firmino helped Gisdol's side come from a goal down to see off Tayfun Korkut's men.
Eugen Polanski, Anthony Modeste and Sebastian Rudy were on target as Hoffenheim made it back-to-back Bundesliga wins for the first time this season.
Gisdol's charges face German, and European, football's most daunting prospect on Saturday when they travel to newly crowned champions Bayern Munich, but they are sure to visit the Allianz Arena in high spirits.
"We achieved a good, disciplined performance - really great. We have again scored three goals and played out a very controlled game," the coach is quoted as saying by Kicker.
"That then Roberto Firmino has extended his contract (is something of) a milestone for us."
Firmino, who has scored 13 Bundesliga goals this season, had been linked to Schalke earlier this week, with the club's coach Jens Keller hinting at an interest in him.
Keller said the former Figueirense man was a player who "could help" his team.
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