The Firelord is the political leader of the Fire Nation and the highest ranking military official. The Firelord reigns over all the generals though he doesn't directly engage in military affairs.
Reputation
This title is well esteemed and the FIrelord is often the wealthiest member of the Fire Nation government.
Power
Many times, the Firelord is one of if not the most powerful bender in his/her nation. This is largely because firelords often have a monopoly of the best training in their country. A lot of resources are specifically dedicated to the Firelord and the Fire Nation culture more directly values having a powerful leader than the Earth Kingdom. The Air Nomads lack such a central leader and for much of history the Water Tribes lack such an authoritative leader that unites them.