After you kill some Sun Midge Swarms and report to Fufulupa, he asks you to find a missing courier.
This quest is only available if you started the game in Ul'dah, and you have completed the quest The Perfect Swarm.
Starting the Quest
Talk to Fufulupa in Horizon in Western Thanalan to start the quest.
Search for Seseli
Your next quest destination is in Central Thanalan. You can either go there on foot by going south, crossing the bridge, and going east; or you can teleport to Black Brush Station; or you can talk to the Chocobokeep in Horizon to go to Black Brush Station.
Seseli is southwest of Black Brush Station. Talk to her to get Fufulupa's letter.
Deliver Fufulupa's Letter
Go northeast to the next quest destination and talk to Leofric, and hand over the letter.
Next Main Scenario Quest
After completing the quest, you will need to be at least level 12 to proceed to the next one. Once you are level 12 or higher, you can talk to Leofric to accept the next quest: Heir Today, Gone Tomorrow.