What's FIFO accounting?
First In. First Out.
Selling/paying tax on my oldest Bitcoins first, instead of my most recent Bitcoins.
eg: I have 5 BTC in my stash that are more than 365 days old. I buy Bitcoin #6 on an exchange for $x. If I turned around and sold that BTC for profit using LIFO (last in-first out), I'd be paying short term capital gains of over 35%. Instead, from an accounting perspective, I sell a Bitcoin from my stash that is older than 365 days... in theory.