Do airport fire departments have to have airstairs?
Sorry unfamiliar with FAA regs, but since Canada's regs are often just rewrites of US or ICAO rules:

In Canada, I cannot find any direct reference to airstairs as required minimum firefighting equipment : Canadian Aviation Regulations (SOR/96-433) section 303.09
there is however a requirement for holding an airport certificate in Canada requiring a safety management system be in place, this involves doing a detailed risk assessment. 

logically having the ability to deplane any aircraft expected to land might be seen as a necessary risk management solution to a number of potential possible issues. There are similar requirements for "air operator certificates", meaning some of the accountability may be place on the airline. Like so much of Canadian law, we will probably not get a definite answer until a large number of people die and the courts get involved.