Firebird2.5-superclassic on Ubuntu 16.04 LTS I installed firebird2.5-superclassic using sudo apt-get install firebird2.5-superclassic and then ran sudo dpkg-reconfigure firebird2.5-superclassic and set the password per https://firebirdsql.org/manual/ubusetup.html. Even after doing this, I cannot gsec SYSDBA. I try running gsec -user SYSDBA -password ********* and i receive  Your user name and password are not defined. Ask your database administrator to set up a Firebird login. 
unable to open database Is there a way to ensure this gets created/fix this?  I cannot seem to find a way.
Ok, so I found functions.sh in /usr/share/firebird2.5-common. Running this creates all of the maintenance scripts and logging files that need to be there. It also creates security2.fdb which stores the security info for users and sets up the password according to the SYSDBA.password file.  You will need to set up two environment variables before running this:

FB_VER


and

FB_FLAVOUR


These need to be set to the point release you are on, ex: 2.5 and the flavor you are running, ex: superclassic.

Note: Mark Rotteveel's solution seemed to work as well, but I think this would be deemed as the more acceptable method.