"An."

In all cases, if you use the term EoFF before the word "thread" and after you use an indefinite article, it must always take "an" because EoFF begins with a vowel, the first letter starts with a vowel sound, and said out loud as either letters or its acronym, it still takes the "e" sound. Unabbreviated, it's "Eyes," another word that begins with a vowel/vowel sound.