Firebrand is a noun that refers to one who agitates in support of a cause.

Our word of the day began in the 13th century as a term for a type — or ‘brand’ — of wood that was used to start or maintain fires. By the 14th century it had become used to refer to agitators for a cause.

My grandmother was a firebrand for equal rights in her day. She would give passionate speeches in support of her causes.