for X5 is:
MINIX NEO X5 20130828 (version 004) 4.2.2 firmware
Release Date: 2013-09-18
from some reason, it is not available on official page, but it you can download it from minix forum.
LINK
IMHO, not much new things according to their changelog:
1.) Added screen shot setting
2.) Added auto hide system bar option
3.) Improved DDR stability.
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