Male-preference primogeniture (oldest son but daughters can inherit if no sons)
Absolute primogeniture (oldest child, whether son or daughter)
Agnatic primogeniture (oldest son with no girls allowed, ever)
Completely up to the discretion of the current Firelord (i.e. the Qing Dynasty)
Male-preference primogeniture but current Firelord can interfere if they wish
Absolute primogeniture but current Firelord can interfere if they wish
Agnatic primogeniture but current Firelord can interfere if they wish
Elected Monarchy (OK this doesn't make too much sense)
Agnatic seniority (oldest surviving brother of current Firelord is heir)
Whichever child is successful in seizing power (early Ottoman Empire)
Partible inheritance (division of the realm between heirs)
Something Else