I'm not very frequent on FI, but this year I've been on 2 different 757-200 with them.
757-200 with winglets - C class 6 rows, AVOD, reasonably comfy seats (still more or less like US domestic F)
757-200 without winglets - C class 4 rows, no IFE/AVOD, old US domestic first seats.

I haven't really examined, but economy comfort seats on non-winglet 757 did not looked convertable to me at all, rather just a usual tight economy. I have impression this particular aircraft was taken from some US airline and used by FI "as is". You can see remainings of Verizon in-seat phone setup on this plane. No idea if this is only airplane in the fleet, maybe not.

Never flown FI eco comfort, can't comment on difference. The sole reason I'm flying FI they have best C product to Reykjavik from Europe