What Firmware to use?

Check serial number - if its end with “xxx-10” - you have 2nd generation device (“Gen2”) and FW v2.xx.xx
If “xxx-00” - its Gen1, FW v04.xx.xx and non-standart “Memory page size” (64kb vs normally 4kb)

For Gen2 i make a “Workaround” for enable “native apps”: WD MyCloud Gen2 - Enable apps install tab + Apps!
For Gen1 - Only Chroot (Debian Jessie + Some of popular software, like Transmission, Minidlna, MC, LAMP pack and other)
For both devices i also make few alternatives - Clean Debian jessie and ported Synology DSM. Check this forum (Use search).