Yes, you can ride other apparatus, but you can only perform duties that you are qualified for (e.g. an EMT that is not a certified Firefighter cannot go into a burning building if they are riding on the Engine).  Firefighters are encouraged to become certified Emergency Medical Responders (EMR) or Emergency Medical Technicians (EMT) and as such could be qualified to staff an ambulance.  Firefighters can also be released as ambulance drivers.