Fed Fire is a group of fine Americans! You'll most likely see them at Worden Parade Field tending to the pass outs and handling random emergencies or fire alarms on the Yard. If you stand Midshipman Officer of the Watch as a Firstie, you might have the opportunity to become quite acquainted with them. I thing I had of streak of four watches where I had to call Fed Fire (nothing super bad luckily).
My advice is to first get established during plebe year. You have a lot of competing priorities and it is hard to manage them even with no distractions. County Fire might be a bit of a tough sell since you'd be leaving the Yard a fair amount at odd hours. Fed Fire might be more workable if they let you volunteer. All you have to do is ask and the worst they can say is no. Even with Fed Fire, it could be a tough sell for liability reasons.
Fed Fire most likely has medical covered by the DoD since their work is dangerous and they have a real potential to get hurt. So if they get hurt, they recover and go back to work. If you get hurt on the other hand, you'll get treated by military medicine, but the Navy won't be too happy if you render yourself non-commissionable due to an injury sustained while being a firefighter. So if you go through the approval process (probably a paperwork drill for your military chain of command and the Fed Fire chain of command), then you can maybe get cleared to do it. Again, gaining approval is the hard part. While you know the inner workings of what being a firefighter is and how a lot of calls are benign, your military superiors are probably looking at it as you running into a burning building or using the jaws of life to save someone right before the car explodes.
My advice is to maybe start an extra-curricular centered around First Responders. Plenty of people are either EMTs/firefighters prior to entering the Brigade and mids also earn their EMT certs through National Outdoor Leadership School (summer training opportunity). You can definitely plan events to go visit first responder units around the area and probably get some good training in.
Starting an ECA isn't too difficult. You just need to find interested people, an officer who will support it, and route the paperwork.