Fireglaives are more of a hybrid unit. Their ranged attacks are less damaging than Blunderbusses (albeit from a longer range) but when they get in melee they are pretty good Halberd units with 10 more melee defence than Blunderbusses and 25 AP vs the 9 of Blunderbusses.

Also, they shoot single projectiles, while Blunderbusses shoot 3 projectiles each with 1/3rd of the total damage. So theoretically Fireglaives would be better at shooting things with lower model counts like Lords or Cavalry since less of their shot would miss, but in my play experience Blunderbusses have pretty good accuracy even against single entities so that's a bit of a wash.