Looks like I got this all wrong. Those cities are only served by their volunteer FD's; the County doesn't have full-time Fire/EMS services. Still find it odd that an area this highly-populated has volleys. Here in Ohio that would be unheard of.
So are any of these stations manned 24/7, or are these traditional volunteers? On my scanner it sounded like somebody was at the stations.
Looks like you're speaking specifically about Prince William County, so I'll shed some light on their situation.
Virtually every apparatus and fire station in the county is owned by the volunteers, however, they're not staffed by the volunteers 24/7. The County's DFRS (Division of Fire & Rescue Services) employs more than 400 full time employees, up to, and including the county fire chief. These full time county employees staff some fire stations during the day time hours only, while they staff other stations 24/7. They also staff several ALS ambulances on a 24/7, which are county-owned and county-lettered. Keep in mind that most of these individual VFD's have a LOT of money, a lot of rigs, and a handful of large stations as well.
The stations that are career-staffed during the daylight hours only are staffed with volunteers at night and weekends. There are both career and volunteer battalion chiefs as well.
A couple of years ago, the county's board of supervisors disbanded one of the county's VFD's and took over their buildings and equipment, so now they have rigs lettered for the county, but still painted in the previous VFD's scheme.
One things that's different in Virginia than every every other state in the nation is that our cities are completely independent of counties. Cities can lie within a county, or outside of a county, but they don't share services unless they're under contract to do so. The City of Manassas (within PWC) still has a very strong and proud volunteer commitment, whereas the City of Manassas Park just down the street is a fully career department. The career staff at both of these departments are employed by their respective city government, but are dispatched by PWC, and work seamlessly with the county folks on a daily basis.
Ultimately, it comes down to the county getting a heck of a bargain with the volunteers providing services and apparatus to the citizens and back to the DFRS. I can't say that there aren't some rocky roads between the career and volunteer staff, but what combination department doesn't have that?
Also, there are still active volunteer departments within Fairfax, Loudoun, and Arlington County as well.
Really want your socks knocked off? Take a look at how the all-volunteer Kentland VFD in Prince George's County, MD (immediately north of DC) is doing over 5000 runs a year, and has been for a decade.
Volunteering in the mid-atlantic is a little different than what you might be used to.
EDIT: I see you might be moving there. Be prepared for a very high cost of living unless you're willing to commute from some distance away!