Pillagers that are attacking a player/villager with a firework loaded crossbow will shoot an arrow instead of a firework. Using the command

 /replaceitem entity @e [type=pillager] slot.weapon.mainhand 1 air 

will remove the pillager's current crossbow in their mainhand and you can give them a firework loaded crossbow with dispensers and activate the dispenser to give a pillager a loaded crossbow. As a result nothing happens, they'll shoot an arrow even though they had a firework in ther crossbow.