Firebird Classic 2.5.x with xinetd on Ubuntu/Debian systems

By default Firebird Classic package uses openbsd-inetd
You can inspect the config cat /etc/inetd.conf the max number of connections per minute is 255
after nowait you can suffix this field with a period, followed by the maximum number of connections authorized per minute for example nowait.10000
localhost:gds_db stream tcp nowait.10000 firebird /usr/sbin/tcpd /usr/sbin/fb_inet_server

To switch to xinetd an easier to use and popular alternative
sudo apt-get install xinetd
This will stop openbsd-inetd and will start xinetd alternative
You can inspect it with
cat /etc/xinetd.d/firebird25

There are a few issues with default config and it will be cleaned in the next firebird-classic package versions
A good config should look like this where i configured an unlimited numer of instances per source (also you can tweak the cps)