NYC residency requirements for FDNY and NYPD are actually quite liberal in the old school sense - members of those departments can live in Nassau, Suffolk, Westchester, Rockland, Orange and Putnam Counties. Nevertheless, there are cases of cops and firemen living across state lines in NJ including a former cop turned mayor who was living in Fort Lee, NJ until recently l ;).
That is quiet a bit different than working in LA where the closest state border is a 4 hour drive away to NV or AZ.
The distance that some of these LA firefighters live away would be like being a firefighter in Staten Island and living in NH or VA.
I would think that most of these LA firefighters are living in other states for tax/quality of life reasons. They can make a lot more money being on the LA fire department than they can in Boise.