If the directory structure from the location of the location of the playlist file to the music files is the same then Firefly will resolve the paths.
e.g. if the path to your music on the PC is c:Documents and Settingsuser nameMy documentsMy MusicArtistalbummusic file.whatever
and the path on the ReadyNAS is c:MediaMusicArtistalbummusic file.whatever
You simply place the playlist file in the Music directory. The structure below that level is the same for both locations & Firefly is able to resolve the path.