For fast, long fire road descents rear suspension is not as critical. More important is tyre selection. You must have a front tyre that you can trust, that you can commit to corners with and not be afraid that it is going to wash out unexpectedly. My personal fave is the Geax Sturdy, which has longitudinal knobs that provide excellent grip and stability and a square profile. They start to slip very predictably, allowing you to drift around corners in full control with no fear of laying the bike down.
When blasting down fire roads it is important to keep your weight fairly central on the bike. Too far back and the front wheel will go light and you may encounter speed wobbles of the front wheel; too far forwards and you may apply too much forward weight and come unstuck if you hit an obstacle.
The gyroscopic effect of the wheels and tyres spinning will keep you upright and aid stability. The heavier the tyres on your bike the greater this effect.
It is important to look where you are going, both in the far distance (mainly to spot walkers and dogs: if you are on a ‘bikes only route’ this is less important) and in the middle distance to spot any possible obstacles.
Pedal as fast as you can and when you reach maximum velocity and your feet can’t go any faster to keep up with your wheels, get tucked down behind the bars and hang on!! (Current FanyLion speed records: off road 44mph, on road 52mph – can you beat them?).
Hucking off a 10ft ledge is just like hucking off a 2ft ledge. It's the same technique, you're just in the air for longer.