It is one of the quirks of the fire service to be sure. It amazes some and displeases others but it always generates buzz.
The fire helmet is constructed to protect firefighters from injury but more than any other piece of equipment, including apparatus, the helmet is most often discussed and personally outfitted. In short some firefighters like to put "bling" on their helmet.
From the shamrock to IAFF stickers, a grinning bulldog or the American flag the helmet sports as many looks as their are firefighters. It's a great source of pride for people on the line.
What matters though, more than looks, is functionality. Will the helmet save your noggin? Bling is fine but breathing is far and away more important.