Subject Re: Is it possible to store firebird DB file on a NAS?
WD my Cloud it enables smb/cifs protocol so a network share can be mapped as drive 
In Firebird 2.5.x is possible with
RemoteFileOpenAbility 1 in the firebird.conf (With a few big warnings if you read the conf file)

you need to take care only the Firebird server to have access to that NAS share (exclusive access)

I recommend an usb3.0/thunderbolt ssd drive for io speed 
If you follow this benchmark

"As you can see, performance with random IO operations shows ~8x better results for SSD drive. We knew from our experience with customers databases that SSD is 30-50% faster with real-word applications, but 8x increase is very high."