Very good question, Canberra. It is standard practice, of course, in normal urban fire stations for them to relax in the station without their full kit. However, their response time to a fire can be anything up to 30 minutes. Ample time in which to wriggle into jackets, helmets etc. However, on an airfield where they have to be able to attend an incident anywhere on the field within four minutes (I believe) it doesn't leave much time. On the other hand, particularly in the height of summer, if you asked them to hang around for a full shift wearing all the clobber, you'd have a bunch of rather upset firemen on your hands. Plus I don't think they'd find it quite so easy to run for the appliances (why are they all called either Dennis or Simon?), so getting the kit on in the cab may well be easier.
If anyone who sees this is an AFS operator or knows one, can they find out the answer please? Enquiring minds would like to know.