Hi!

TCP connections are limited in xinetd.
Edit /etc/xinet.d/firebird file

service gds_db
{
 disable = no
 flags = reuse
 socket_type = stream
 wait = no
 tcps = 500000 10
 user = firebird
 per_source = UNLIMITED
 instances = UNLIMITED
 server = /opt/firebird/bin/fb_inet_server
}