There is reference to installing Firebird 2.5.x on Ubuntu on the website at https://firebirdsql.org/manual/ubusetup.html  but this information is very old.  The repository that is referenced does not seem to work with Ubuntu 18.04.  When I attempt to add it to my sources and then update, I get the following error:

E: The repository 'http://ppa.launchpad.net/mapopa/ppa/ubuntu bionic Release' does not have a Release file.

N: Updating from such a repository can't be done securely, and is therefore disabled by default.