Core77's David I. Seliger has a nice retrospective on the design of American firefighter helmets from the 1731 helmets designed by Jacobus Turk for the Fire Department of New York to the contemporary thermoplastic models. These modern helmets, Seliger says, have plenty of room for improvement, and "Redesigning the fire helmet - both a marketable product and a symbol deeply ingrained in American culture - to satisfy firefighters' physical, mental, and emotional needs could be the industrial design challenge of a lifetime."
Three main types of helmets are currently used in the United States. The "leatherhead" is just that--constructed almost entirely of leather. The leather's resistance to heat actually rivals that of modern composites. The large brim that dips down in back is designed to keep hose water dripping off of the ceiling out of the face. The leatherheads are used in fighting regular fires.
The second type is the structural helmet, a more streamlined version of the leatherhead, constructed of thermoplastics or composites. These helmets are used for structural collapses and extrications. Goggles, instead of a face shield, often sit on the front of the helmet.
The last type is the European-style helmet, which looks like a cross between a motorcycle helmet and something from Star Wars. The design is purportedly much more comfortable and practical. But, as one firefighter put it, "I've been involved with fire departments in four states...and in seven years have seen a grand total of two [European-style] helmets... these guys were... universally afraid of being mocked for wearing something new, different or 'unfashionable.'"