The mods available in this thread are free of charge and provided by the community. All of these are built on the official FiiO firmware.
When you flash a firmware
I expressly linked to the post where the firmware was posted in addition to the direct download link. If you like what you see and use, I strongly suggest that you click on the "post" link and go give reputation to the modder. If you have opinions about the firmware, I encourage you to tell it to the modder. It's not guaranteed that they will change their mod but it could give them ideas for future firmware mods.
FiiO X5II Firmwares
This thread contains no firmwares for the FiiO X5II (Second generation) player. These can be found in this thread from @x RELIC x.