Firebird doesn't register itself as a service, so no GUI for
activating it. I'm posting here the solution for others.

I copied /etc/xinetd.d/ftp onto /etc/xinetd.d/gds_db and made some
modifications:

"service gds_db" instead of "service ftp", got this from /etc/services

"/Library/Frameworks/Firebird.framework/Resources/bin/fb_inet_server"
instead of "/usr/bin/ftpd" (for Firebird version 1.5)

removed the remaining lines (don't know if this is necessary / wise).

It worked, after activating FTP sharing. Since security is not a
concern in this particular case, for me it's ok to leave an extra
service running.

THX,

Lucio.