A dense, bushy shrub, Yukon Belle® firethorn (Pyracantha angustifolia 'Monon') naturally grows to a height of 8 to 10 feet. Repeated pruning, called "heading back", to encourage horizontal branching promotes a lower, sprawling plant habit. But it's still difficult to keep it under 3 to 4 feet in height. Perhaps a lower-maintenance choice would be cultivar Shawnee, which matures to only 3 feet tall and 6 feet wide.
Put on thick, protective gloves. Avoid getting punctured by this shrub's thorny stems as you work.
Prune back upright branches on the Yukon Belle®. Make the pruning cut in a crisp, clean motion of the hand pruner blades 1/4-inch above a horizontal branch you wish to keep. Select horizontal branches that are 6 to 12 inches below the height you tolerate for a ground cover. For example, if you want the pyracantha to be no taller than 4 feet, the horizontal branches retained must be no higher that 3 feet above the ground on the main plant trunks.
Examine the entire shrub, continuing to prune away upright-growing branches, retaining long, arching horizontal branches that already sprawl and reach the ground. If possible, reduce the number of pruning cuts made, because the plant's natural response will be to sprout new upright, suckering shoots.
Monitor the shrub through the rest of the growing season, repeating Steps 1 and 2 to continually remove branches, twigs and sprouting shoots that grow upright rather than horizontally.