Note: Do not rely on this information. It is very old.
FF, the sixth letter of the English as of the Chalcidian and Latin alphabets is derived from the Phoenician Van, which probably is taken from an Egyptian hieroglyph representing the horned asp or cerastes. It appeared in Greek as the digamma, which, however, was soon lost in most Greek alphabets, and occurs only in early inscriptions, and as the numeral for 6. The Roman F had probably the English sound, whereas the Greek digamma was probably a W or intermediate between that and V. In Anglo-Saxon and in Welsh F has the sound of V, the modern F sound being represented by ff.