Dalmatians!

Why are Dalmatians the continues "fire-fighter" dog?  Well, there are several reasons:

Dalmatians are exactly the right build for the job; they area very physical and muscular, they are long, lean, and able to run great distances without tiring, and they are easily able to be spotted (no pun intended) in a smoky area.

 Dalmatians are a breed that has long been a friend to horses, which firefighters used to use to pull their steam-powered apparatuses.  For some reason they have a strong calming affect on horses; Dalmatians and horses form a deep brotherly bond.  Dalmatians used to be used to calm horses in stagecoaches; they would run along side the stagecoach, just like they began to be used for horse drawn fire fighting apparatuses. Also, the Dalmatians guard the horses, sleeping with them and hanging around them all day.

 Dalmatians are just a really loyal breed of dog in general.  Through tradition and experience, they have become a firefighters companion and a symbol of fire service.

Also, a small reason I did not see mentioned anywhere but that came to my mind was that they have relatively short hair that does not shed, which would make them easier to handle at the fire house and at the fire.