/share/flash/data/public/ is an invalid log file. This is a path, not a filename.
/share/flash/data/public/firefly.log would a a nice filename.
To be able to have firefly log there, you need to create the file, and then give it 755 rights, and set the owner to be guest:everyone.
touch /share/flash/data/public/firefly.log
chmod 755 /share/flash/data/public/firefly.log
chown guest:everyone /share/flash/data/public/firefly.log
Would do the trick.
This gives you read permission, and firefly write permissions. 😉
So then after you started firefly, this would come in handy:
tail -f /share/flash/data/public/firefly.log
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