Firstborns are seemingly ancient Vampires who were sealed away in the catacombs where Bloody Mary's body was kept. After Bloody Mary was revived, they were released to help her create havoc in New Marais.
These vampires have the ability to disguise themselves as humans, the only distinguishing feature being, when Cole uses Vampire Sense, their heart, veins and vessels are not visible since there is no blood flowing through them. When attacked in this state they mutate into their vampiric form, however staking them in their human form kills them instantly.
When in their Vampiric form, they can withstand two stakings (Shadow Stake takes them down to half health) with a third being required to kill them. They have health meters accompanied with them, meaning they're mini-bosses just like the Ravagers and Crushers from Infamous 2.
They are the only enemy Cole encounters during the course of Pyre Night who are docile when first encountered (while some are in their human form, several Firstborns already in their vampiric form are encountered in missions) and must be either intentionally or unintentionally aggravated into combat by attacking or attempting to bite them. Once they mutate into their vampiric form, they remain on the map in the general vicinity of where they were aggravated until either a mission is started or Cole enters the catacombs.
Also after Cole has attempted to bite or attack a hidden Firstborn, the Firstborn will mutate into their vampiric form and immediately start biting and infecting pedestrians in the current vicinity. The more populated the area is, the more people the Firstborn can infect meaning the more Firstborn victims Cole will have to fend off.
If Cole manages to stake and kill the Firstborn with infected victims still alive, they will be cured of the vampire infection and return to normal humans, just like how Cole was cured when Bloody Mary was killed at the end of story.