partial success …
I just upgraded from flac-1.1.2 to flac-1.1.4 and now firefly reads the plugin correctly. I actually see the flac file on my SB.
However, I still can’t play it: firefly fails with
Session 0: Streaming file 'frog.flac' to 192.168.1.69 (offset 0)
Illegal instruction
wavstreamer: Can't read wav header.