It's Russian. Because the Greek 'theta' looks like the Cyrillic 'f' rotated at 90°, a lot of Russian names have an 'f' where the corresponding western name would have a 'th'. See Marfa, Dorofey, Feodor, Afanasy, etc. Or, for that matter, Esfir :)
Ivayla, this is most interesting! I'd assumed that it was a pronunciation issue, as many languages don't have the -th- sound and often substitute a -t- or -f-. Thank you.