Proposed Canon
First Mentioned: IPTSF Text 6
Fireman is a unit. It is one of the unit types used by Haffaton. Its appearance and stats are not known yet, although the name and shown abilities suggest that firemen are humanoid and may belong to the race of men.
Firemen can engage a fire instead of enemy units.
A squad of mid-level firemen from Haffaton ambushes Goodminton's column of siege engines.IPTSF Text 6 Later inspection discovers that firemen were armed with pickaxes and shovels, and had digging special ability. Wanda uncroaks the firemen and uses their ability to dig through city walls to recapture Goodfinger from Haffaton.
As Firemen can engage a fire, they likely get some form of bonus to 'fighting' the fire out that is unavailable to conventional troops.
It is also possible that they posess some sort of "flamethrower" special whereby they may deal extra damage to siege equipment or other related items. This is supported by the references to their "burning" Goodminton's siege.
 Real World References
Firemen refer to firemen from Real World. However, Haffaton's firemen burned Goodminton's siege engines while firemen of the Real World extinguish fires.
Possibly also a reference to Ray Bradbury's Fahrenheit 451, wherein Firemen enforce the national censorship regime and incinerate all discovered books.