To create your own Firefox Sync server you need this file -> click. If file is not accessible try this or contact me.
( There is a contact form somewhere here )

So when you got all the needed files click away to your NAS station.
First make sure that you have installed myphpAdmin and mySQL is enabled. ( You can get those in Package Center under available tab )
After that log in to your myphpadmin console and create new DB with name FFsync ( or something else )