Yeah, that's Y3 Firefly. As Katrine said, curly haired ponies in general have longer hair and the fibre is slightly different if it was originally curly. Though the curl can drop out, it's still identifiable in loose ponies because of that reason. A similar thing happened (in reverse of course) with the 2 releases of Cherries Jubilee in the UK.
I think the year 3 versions of the four ponies (Firefly, Medley, Applejack and Bow Tie) are probably more rare than their original releases just by dint of being re-released. I am guessing from Katrine's post that curly Firefly might be the one sold with the Fable set (?). If so I guess in her case she might be more common in those areas. I am not sure there's a price difference either, because not everyone is interested in getting all variations...but in general I would say the curly haired versions are harder to find.