The fireworks are because (a) the player is running the hijack kernel, and (b) a custom boot animation has been stored in the player's flash RAM.

FAQ entry on what controls the boot up logo sequences.

FAQ entry on how to change the custom animated boot logo.

Note that the original factory boot animations exist in the boot logos section of riocar.org . You can drop those into Jemplode's animation editor so that the player plays the original factory boot animation even when you're running the hijack kernel.