On December 2, Firkser scored his first NFL career touchdown on a 12-yard reception from Marcus Mariota in a 26-22 Week 13 victory against the New York Jets. Firkser finished the 2018 season with 19 receptions for 225 yards and a touchdown. Firkser began the season as one of four tight ends on the Titans opening day roster along with veterans Delanie Walker, Jonnu Smith, and MyCole Pruitt. For the first six weeks of the regular season, Firkser played sparingly and was even a healthy inactive in Week 4. During that six-week span, he caught only one pass for 25 yards. After Walker was later placed on injured reserve, Firkser played a larger role in the Titans offense. During Week 10 against the Kansas City Chiefs, he caught three receptions for 36 yards and his only touchdown of the regular season as the Titans won by a score of 35-32. Firkser finished the season with 14 receptions for 204 yards and a touchdown. Ironically, Firkser's first two career touchdowns were against the Jets and Chiefs - the two teams that had previously waived him. The Titans finished the regular season with a 9-7 record and were the sixth seed in the AFC playoffs. Firkser played his first postseason game on January 4, 2020, against the defending Super Bowl champions, the New England Patriots. In that game, he caught two passes for 23 yards and a touchdown in the Titans 20-13 upset victory. With the touchdown, he became the first player from Harvard to score a touchdown in an NFL postseason game. Firkser's final reception of the season was also the final touchdown of the Titans 2019 season. He caught a 22-yard touchdown pass in the fourth quarter of the AFC Championship Game in a 35–24 loss to the Chiefs. On February 13, 2020, Firkser signed a one-year extension with the Titans. Firkser volunteers with his former high school football coach to help train and give advice to aspiring young football players in his hometown community of Manalapan. He enjoys pencil drawings, jigsaw puzzles and playing online collectible card games. His brother, Josh, played football at Wagner.