Strongpoints

[triangular_cube]

From 12.43

If the hex contains defending units that receive first fire and units which do not, the defending player may choose to take First Fire with only the units able to first fire (the others don't fire), OR use all his units and resolve the assault simultaneously with the attacker.

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One could read that as the other units dont fire at all (instead of not firing during FF) when the first fire is elected but I don't think that is the intention of the passage. 

To me it was defenders option

A) First fire with eligible units, normal fire separately with ineligible units.

OR

B) Normal Fire with all units

Just matters how it columns out to decide which is better.