Pyracantha are grown in a number of different ways, stand alone shrubs; hedges; wall plants; screening and small trees, so it is as well to know a little about how they flower and subsequently berry, before the act of pruning or trimming.

They have the advantage of being fully evergreen shrubs that have masses of creamy white flowers in early summer, with masses of orange to red berries from late summer. One wrong pruning session can wipe out flowers and berries for at least a year!
Firethorns flower on stems grown in the previous year or older. They do not flower on new stems made in the current year. Typically, new stems start to emerge in May/June immediately after flowering. Those stems will then grow for the rest of the year maybe 18-24in (45-60cm) long producing dormant buds that hold the flower trusses for the next year. If you prune them off, you are pruning away the flowers for the following year.
Suitable for pruning all common varieties, including 'Mohave' 'Red Column' coccinea types and atalantioides.
Pyracantha berrying on stem grown the previous year.
Most varieties will flower the full length of the shoot, so it is possible to prune the shoot back halfway, and still leave plenty of flower bud developing for the following year.