DBD::Firebird is based on DBD::InterBase, the interactive configuration feature was a default in DBD::InterBase and we chose to keep it as an option.
For "the full path to the Firebird installation" the FIREBIRD_HOME environment variable is checked first, on failure, a sub tries to locate Firebird in some default paths, but "/usr/lib/firebird/2.1" is not among them, that's why you had to enter the correct path. By the way what OS is yours? (if GNU/Linux then what distribution?, because other search paths can be easily added).
Suppose the test that failed is rt72946.t (it's a known bug) or where other test that failed?, unfortunately there is another bug, recently discovered, see https://github.com/mariuz/perl-dbd-firebird/.