While Subway made a fortune touting low calorie counts, Firehouse is known for selling hefty sandwiches for big appetites.
The quick-service restaurant chain boosted sales by $110 million last year, according to QSR Magazine. Firehouse has nearly 1,000 locations in the US and plans to double its footprint by 2020.
It's the second fastest-growing franchise in America, according to Nation's Restaurant News.
Meanwhile, Subway has been plagued by negative sales and a scandal involving its former spokesman Jared Fogle.
The secret to Firehouse's success could be its unapologetic approach.
Rather than boast freshness, Firehouse focuses on advertising flavor. The brand's online menu is focused on "bold sandwiches" and satisfying appetites. The subs are piled high with meat and cheese.
Firehouse Subs
Chris and Robin Sorenson, founders of Firehouse subs
With its vegetables and lower calorie counts, Subway arguably invented the idea of "fresh" fast food two decades ago.
But while Subway stayed the same, better competitors got into the space.
Chipotle offers food that is raised without fillers or antibiotics and is prepared fresh in stores. Firehouse Subs and Potbelly offer elevated ingredients and side dishes such as gourmet kettle chips and potato salad.
Americans who once praised Subway's low-fat offerings are now concerned the chain's lunch meats and sauces are overly processed with fillers and additives.
"What Americans see as healthy has evolved," Drew Harwell at The Washington Post writes. "Subway hasn't."
Ashley Lutz contributed to this story.