The ESP needs a FAT filesystem and it should be of type "ef00" in the output of
Code:
# gdisk -l /dev/adaX
(Replace X with the correct identifier for your drive.)
It could be that the disk has an MSDOS partition table ("MBR" type) — not all UEFI firmware implementations will support UEFI booting from such a device despite it being part of the official specification. A GUID partition table (GPT) is the only way to guarantee compatibility.
Apologies but I'm not sure about the gpart(8) output; it's been a while since I tried FreeBSD (as you probably realised from my erroneous post in your other thread).