What does the Partisan Marksmanship Action mean?
Does 'firing with'
The full quote is firing with a Partisan Leader. That means the Partisan Leader is participating in the Fire Attack - either as the sole unit firing or as part of a fire group. So if the Fire Attack Total includes the Leader Unit - Partisan Marksmanship can be played.
It does not apply if only the weapon assigned to the leader is part of the Fire Attack total. In that case, the weapon is firing, not the leader so the prerequisite is not met.
Nor does this action apply if a leader activates other units who then participate in a fire attack without the leader. Again, the leader is not firing and the action's prerequisite is not met.