If you create a timestamp column and call SQLDescribeCol the display size is returned as 24 but the driver can return 29 characters for some dates:

1998-05-15 00:01:00.100000000

The tests in DBD::ODBC fail because DBD::ODBC believes that 24 and bind with a column length of 25 but then they get data truncation error. I had to work around the problem in DBD::ODBC by adding the following to dbdimp.c:

        /* Workaround bug in Firebird driver that reports timestamps are
           display size 24 when in fact it can return the longer
           e.g., 1998-05-15 00:01:00.100000000 */
        if ((imp_dbh->driver_type == DT_FIREBIRD) &&
            (fbh->ColSqlType == SQL_TYPE_TIMESTAMP)) {
            fbh->ColDisplaySize = 30;
        }

which you can find in the subversion trunk for DBD::ODBC.

The display size should be set to 29 or you should not be returning all those digits for the fractional seconds - I cannot say which as I don't really know Firebird.