What do firefighters do on ordinary days?
Calling them "firefighters" is accurate but they can respond to all sorts of incidents, not just fires (in the USA, even outside airports it's common for firefighters to respond to purely medical emergencies). On top of that, there's also training, maintenance and keeping physically fit.

This is from a firefighter at Hobart Airport in Australia:


  The aviation firefighters do more than wait for airplane fires, they
  also attend to other incidents, such as motor vehicle accidents, fuel
  spills, hazardous material incidents and first-aid in the terminal.
  
  [...]
  
  "A lot of our working day is routine and there are different types of
  things that we need do every day regarding our equipment, servicing
  all of our equipment, checking it over ...," he said.


And this is from one at Heathrow:


  We don’t really have an average day. We attend to all sorts of
  incidents. We attend to anything that is life-threating at the airport
  or on the roads around it; this could include a heart attack air or
  land-side, a drug mule incident where a passenger has transported
  drugs within their body and has ended up in a serious condition, or a
  car crash that we have to cut a victim out of.
  
  We all have set duties to do every day and night including practical
  and theoretical training, testing and inspections, and cleaning of
  vehicles and equipment. The crews have to keep physically fit so we
  give them time for physical training and provide facilities to do
  this.