I have found a solution with this /ffp/start/syncthing.sh:
#!/ffp/bin/sh
#PROVIDE: syncthing
#REQUIRE: LOGIN
. /ffp/etc/ffp.subr
name="syncthing"
command="/ffp/bin/$name"
required_files="/ffp/var/syncthing/config.xml"
syncthing_flags="-home=/ffp/var/syncthing"
run_rc_command "$1"
With a copy of the Syncthing database in /ffp/var/
Syncthing launches correctly, but then I cannot be any more connected with Putty or Filezilla (refused connection), and the other process (Mediatomb) don’t launch at startup !