The FBAR instructions may be unclear on this point; but the answer to your question is that you should use your current mailing address (or at least an address where you are currently able to receive postal mail).  The reason for this is that the whole point of gathering your address data is so that the US Treasury Department will know where to reach you and contact you (if they have questions, for example).  For that, they need to know where you are now . . . not where you've been in the past.

The same line of thinking goes for entering a return mailing address on a Form 1040 tax return.   You don't want to use the address of where you were living on December 31st (i.e., the end of the tax year), for instance.  Instead, if you've since moved, you want to use the address of where you are now, as of the date you file your tax return.

Hopefully, when you stop for a moment and think about the logic of that situation, it will make sense.

Thank you for asking about this important clarification.