Crassus didn't own the fire department. Crassus owned a bunch of slaves who he had trained to put out fires.

Crassus didn't charge for extinguishing fires. If your house was on fire, Crassus took his team and negotiated to buy your house. If you sold it to him, he'd put out the fire. There was no flow of money from you to Crassus under any circumstances. (But he could get a low price, because the value of your house was constantly dropping while it burned.)

In imperial times, private firefighting groups were illegal since they were viewed as a potential source of rebellion. (Using your own slaves to put out fires on your own property was of course fully legal.) It's in my mind that Crassus benefited from similar laws, but he was active during the Republic. Maybe someone else knows more about the precise timings.