You can install the firmware by doing the following:
download the firmware updater for the device if you haven't already
insert your recovery disk
flip boot mode switch to "USB"
turn on the device, wait several minutes for it to finish booting into EM mode
enable debug mode for tsupdater:
https://buffalonas.miraheze.org/wiki/Enable_Debug_modeuse tsupdater to install the firmware.
**if you don't care about the data on the device you can use the re-partition/re-format options, otherwise you'll want to stick to just forcing a firmware install to try to recover with the data intact.