The Firebrand is a techique learned by Mario at Oho Oasis from the Sun God in Mario & Luigi: Superstar Saga. It can be used by switching over to the Hand icon and holding A until it is fully charged. This matches Luigi's Thunderhand technique.
The Firebrand is mainly used to help light unlit torches and ropes. With learning the Firebrand, Mario and use a Bros. Move with Luigi called Fire Bros. Now being elemetal, the Firebrand can be used to attack certain enemies for Critical damage such as blue Piranha Plants. Though, it will heal fire-based enemies like red Piranha Plants so beware. Also, once after learning the newer technique from Gigi and Merri, Mario can used the Firebrand on Luigi to run at a very fast rate knocking over Ankoopas and running past hazards.