You do not half firepower for long range, but instead you would roll your normal dice but only hit on "6".
Basically, as the enemy unit moves into a hex within range (close, normal, or long) and LOS of the op fire unit, then player who is moving would ask you if you wish to use your opfire. You have the option of 1) waiting, 2) firing. The other play may continue moving their unit after 1) you have done nothing (or) 2) the opfire has been resolved.
If you did not attack the unit, and the enemy decides not to move that unit any further, then you have lost your opfire oppurtunity for that unit. But yes, sometimes it is better to wait if you believe that the enemy unit is going to continue advancing.
For instance, an enemy unit moves into LOS of your opfire unit, however, that enemy unit is at long range, so you decide to wait. The enemy unit then advances another hex closer and is still in LOS of your opfire unit. However, since they moved closer, they are now at normal range so you decide to use your opfire and fire at that unit. Now you will score hits on that enemy unit on "5" and "6" (normal range) instead of just on "6" (long range).