At present, fire crews administer basic medical care as and when it is needed.
Under the new plan, launched on Tuesday, crews will get additional training and will be deployed to treat casualties in instances where they are closer than paramedics.
Ambulances will still be sent, but firefighters will help people until they arrive.
All three fire services in Wales are taking part in the project, which runs until June, in conjunction with the Welsh Ambulance Service NHS Trust.
South Wales Fire and Rescue Service has begun, with the mid and west Wales and north Wales services to follow suit in the new year.
Greg Lloyd, head of clinical operations at the Welsh Ambulance Service, said: "If our fire service colleagues can get to a scene before one of our ambulances they can begin to deliver life-saving treatment - that's only going to improve that patient's chance of surviving."
Firefighters will be trained in basic life support, CPS and how to treat a casualty who is bleeding heavily.
North Wales Fire and Rescue Service's assistant chief fire officer, Richard Fairhead, said: "We would like to reassure residents across Wales that there will be absolutely no reduction in emergency response and service delivery, either from the fire and rescue services across Wales or from the Welsh Ambulance Services Trust, during the pilot period."
Fire engines will not be sent to these calls in north Wales. Instead, a car, staffed with two firefighters and containing medical equipment, will be deployed.
It will not have blue lights and will drive to the scene within the speed limit.
Incidents where they could be used include cardiac arrests, people who are unconscious or choking and casualties suffering from "catastrophic bleeding".
Across the UK, 43 fire services have taken part in similar pilot schemes and a review will be held at the end to determine its success.
Firefighters will be sent to treat people at the scene of accidents before paramedics arrive, in a pilot scheme.