He played college football at Harvard and signed with the New York Jets as an undrafted free agent in 2017.

[1] He has played in the NFL for the Tennessee Titans, Atlanta Falcons, Detroit Lions, and New York Jets.

He is Jewish and he and his family belonged to Temple Shaari Emeth in Manalapan, where he celebrated his bar mitzvah.

He was also named the Shore Conference ‘A’ North Division Offensive Player of the Year and made the all-state team.

During Firkser's three-year high school career, the Manalapan football team went 30–6 and won three consecutive ‘A’ North Division titles.

Scouts also praised him for his ability to quickly get into pass routes and adjust to errant throws.

However, scouts negatively viewed him as possessing just average size and questioned his strength as a blocker.

[23] During a Week 13 26–22 victory over the Jets, Firkser scored his first NFL career touchdown on a 12-yard reception from Marcus Mariota.

[25] 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.

After Walker was later placed on injured reserve,[26] Firkser played a larger role in the Titans offense.

During Week 10 against the 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 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 road victory in the Wild Card Round.

He caught a 22-yard touchdown pass in the fourth quarter of the AFC Championship in a 35–24 road loss to the Chiefs.

[31][32] During Week 6, Firkser recorded his first career 100-yard receiving game in a 42–36 overtime victory against the Houston Texans.

Firkser finished the game catching eight of nine targets from Ryan Tannehill for 113 yards, and added a touchdown in the first quarter.

[36] During Week 11 against the Texans, Firkser scored his first touchdown of the season when he recovered a fumble in the end zone.

[37] During a Week 17 34–3 victory over the Miami Dolphins, Firkser caught three passes for 24 yards and his first receiving touchdown of the season.