Crassus created the first Roman fire brigade, and many years later, Augustus took the idea and formed the Vigiles, a group of slaves serving as firefighters. However, I can't find any source confirming that Crassus's fire brigade was also composed of slaves. This seems like a reasonable assumption, since Plutarch mentions that Crassus owned hundreds of slaves, but I was wondering if there was a source explicitly stating that the fire brigade was also slaves.
Plutarch does say it, though you have to understand the context:
And besides this, observing how natural and familiar at Rome were such fatalities as the conflagration and collapse of buildings, owing to their being too massive and close together, he proceeded to buy slaves who were architects and builders. Then, when he had over five hundred of these, he would buy houses that were afire, and houses which adjoined those that were afire, and these their owners would let go at a trifling price owing to their fear and uncertainty.
This is the brigade people refer to when they talk about Crassus' men.