The Linksys NSLU2 (slug) presents arguably the most mature Network Attached Storage (NAS) platform for Firefly. It is a very flexible device with stable firmware, a large and organized supporting community, and lots of available software packages. The community has created modified firmware, known as "Unslung", which allows you to add extra packages, including Firefly.
Further reading
NSLU2 Installation gives detailed instructions on how to set up Firefly on an NSLU2.
http://www.nslu2-linux.org/ is the location of the NSLU2 modification community.
Mt-daapd package article on the nslu2-linux.org wiki gives their description of Firefly and how to use it on the NSLU2. (Mt-daapd is an old name for Firefly.)
... the Media Server.