I had a similar problem, this is due to xinet

From a fresh install:

sudo su
apt-get install xinetd
apt-get install python-software-properties
add-apt-repository ppa:mapopa
apt-get update
apt-get install firebird2.5-classic                 
netstat -an | grep 3050             #shows the problem: not binding to 0.0.0.0
nano /etc/firebird/2.5/firebird.conf
    #comment out all RemoteBindAddress = XXXX
nano  /etc/xinetd.d/firebird25
    #set  bind                    = 0.0.0.0
/etc/init.d/xinetd restart
dpkg-reconfigure firebird2.5-classic
netstat -an | grep 3050             #shows the fixed as binding to 0.0.0.0