I want to make a staff that shoots purple fireballs which lights mobs on fire

Make the textures for the fireballs. Make a ranged weapon as the staff. (You can get rid of the bow sound effect if desired.) Make an item with fireball texture. Set item used as bullet for the staff to fireball item. Make a procedure for "when bullet hits living entity" in staff. Attach "set event/target entity on fire" block. Save. I hope this was helpful