EDIT: Through June 30, they've upgraded their signup freebie to a free small sub with any purchase! Simply join their Firehouse Rewards or download their app. Plus I believe the birthday offer removed the purchase requirement, so it's should be a straight up free medium sub now.
Given that I've eaten here a million times, I was surprised to see I didn't have a Firehouse Subs birthday freebie post published. I always found they made Subways sandwiches look like they were made in an actual NYC subway.
Signup Freebie: Free large drink with a sub purchase.
Birthday Freebie: Free medium celebratory sub!
One nice perk of their new freebie is that you can redeem it not just on your birthday but on the 7 days after it as well. And of course I've added this to the birthday freebies list!