The instructions state (on page 6) the following:
"The geographical location of the account, not the nationality of the financial entity institution in which the account is found determines whether it is in an account in a foreign country. Report any financial account (except a military banking facility) that is located in a foreign country, even if it is held at an affiliate of a United States bank or other financial institution. Do not report any account maintained with a branch, agency, or other office of a foreign bank of other institution that is located in the United States."
This suggests to me that it's the branch address.
