I just wanted to post a huge thank you here to cstern for posting such clear and detailed instructions. Using them, I was able to get Firefly up and running on my Synology DS-106j quite easily.
I did find that the latest nightly build (1696) had some stability issues and crashed when accessing large playlists, so I went back to 1586, and it seems to be working fine now. (You can install an earlier version by adding the “-force-downgrade” argument to the ipkg install command noted at the beginning of this thread.)