Some not very busy airports can have no even a minor incident for years and accident probably won't happen in a major airport during several decades. Training of course is done sometimes. I don't know how often though. This has to be the most boring or the best job if you wan't nothing to do. I think you can spend all your career without putting out single real fire or doing real rescue. So what do firefighters do day to day?
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.