Getting a EMT-P rating (or any EMT rating) while being a firefighter depends on the department and opportunity for clinical time.

The book learning in EMR through EMT-P isn't what is difficult or hard to obtain. It is the clinical time, which will require at the EMT-P level quite a number of hours of "ride-along" in an ALS ambulance service (or with a firefighting service that has ALS ambulance service with paramedics), with other EMT-P's acting as preceptor(s) and observing and approving the student's practice in the field. This practical experience issue is the tough exposure to get, especially as medical practices change (eg, doctors are now trying to reduce the number of endotrachial or "ET" airways applied to patients, and this reduces opportunity for these advanced airways to be seen in practical experience). 

This will also be the issue to keeping a EMT-P license: the clinical time and procedure count every two years.

Getting paid for becoming a EMT-P requires working for a fire department that pays well. Being a EMT-P outside of a fire service is often pretty poor pay, and this is one of the reasons when, given a choice between getting my EMT-P or my RN, I chose RN. The paramedics of 20+ years experience told me "if you wanna be paid, go to nursing school. Same two years as paramedicine, but you get paid for your trouble."

Keeping my RN license is simple compared to being a paramedic: I work as a RN for at least 500 hours a year, and I send in my $55 to the state, and I'm renewed. Renewing a EMT-P license is a lot of CEU's and a bunch of IV starts, advanced airway application and other requirements which change depending on the state.

Is it possible to get a EMT-P while being a FF? Maybe, depends on the department. Will he need to get some level of EMT? Most likely, and most firefighters are at least a EMT-B (Basic) and many are EMT-A. In this state, many of our firefighters are EMT-I, which is one level below EMT-P and still an ALS level of response, but I think only six states still have a EMT-I rating.