The simple answer is that the firing happens at any point in the order that the player cares to. Remember though that
FtF roll conflict care not at what point in the order the conflicting orders occur, just that conflicting orders occurred. "I'm going to shoot you here, but I'm shooting you back there" is still resolve as a
FtF roll at their designated distances.
"When shooting, the player can always choose the moment during the execution of his order in which to attack. The most usual choice is a point in which the situation is the most advantageous: a clear
LoF, less cover for targets or optimum weapon distance. This is also applies when using an
ARO to shoot."
http://infinitythegame.wikispot.org/Ballistic_Skill?action=show&redirect=Ballistic+Skills
Moral of the Story: Do NOT leave cover till the models with
LoS are good and dead unless you care not for your cover bonus.