The energy transformation of a firecracker is Chemical to Kinetic. The energy in the firecracker chemicals is freed by ignition and the chemicals breakdown to simpler components. The chemical energy will then transform to motion energy.
Check this site out: http://matse1.mse.uiuc.edu/energy/a.html it demonstrates exactly what vincentA84 is talking about. Once the energy is transferred from Chemical to Kinetic is is measured in Joules. Very interesting indeed.