We're getting off topic a bit here but a Jacobs is often adjustable through a lo-med-hi switch. Fire trucks aren't very heavy and the transmissions are speced for the weight and the fact that the drivers are trying to get there fast, not row through a 13. They probably would not have a very aggressive engine braking setup.
It also depends a lot on ratios and engine RPM. Because the Jake turns the engine into a big air compressor, it will suck up more HP at higher RPMs, and will offer a greater mechanical advantage in lower gears.
On a heavy-spec truck tractor, or a heavy spec dump that's setup to pull a tagalong, you would be able to use very aggressive jake brake settings - you'd have enough gears to make use of it, low axle ratios, a driver that can float gears on downshift, and enough weight to need it. Running lightly loaded in the hands of a bad operator (or one goofing off) you can skid the drives pretty easily.
Ongoing Defender/Series inspired crew cab project
Diesel-swapped 1994 80 series Land-cruiser