In the MP Reveal Trailer for Call of Duty: Modern Warfare, you may have noticed that theres a scene where the character is climbing up the ladder and turns around and fires the weapon at an enemy. That mechanic was not fully tested out during our hands-on event in LA with Call of Duty: Modern Warfare, but Infinity Wards MP Design Director has confirmed on Reddit that there is in fact the mechanic to fire your weapon while climbing a ladder in Call of Duty: Modern Warfare.
Source: N4G PC Modern Warfare features a mechanic to fire your weapon while on a ladder