No much difference.
EFL stands for "English as a foreign language", while ESL stands for "English as a second language".
From my experience, ESL is used by far wider than EFL. Actually, Google confirms it:
Results 1 - 30 of about 20,100,000 for ESL
Results 1 - 30 of about 3,090,000 for EFL
Another point, my Microsoft Word Spell Checker doesn’t know such abbreviation as "EFL", but it doesn't underline "ESL".
If anybody knows a better explanation, please share it with us.