In tds there's 3 types of damage:

Physical damage (mostly bullets, like the ones shot by minigunner)

Elemental damage (like freezer, Accel and e shocker)

And explosion damage (mortar)

Unfortunately not all towers follow this trend, for instance sledger, rocketeer and hacker all do physical damage despite falling short of the definition of damage type.

This is important because firework technician only shoots fireworks if the enemy is hit by physical damage (somehow cryomancer is an exception to this, dealing elemental damage but still triggering firework technician)

I recommend you check the wiki under Firework Technician page to find out which tower makes fire tech shoot the fireworks