Again, you can make a firearm scale with dexterity (or any other stat) by setting baseDamageStat when you define the FirearmAttackComponent. You just can't change it dynamically like you can with other attack components.
(If you really do need to be able to change it dynamically, and don't want to wait for this bug to be fixed, you could work around it by defining five versions of each of your FirearmAttackComponents, one for each stat and no stat, and just switch between them in the ItemComponent as appropriate.)