No problem, at firebird.conf you find this:
# ----------------------------
# Engine currently provides a number of new datatypes unknown to legacy clients.
# In order to simplify use of old applications set this parameter to minor FB
# version datatype compatibility with which you need. Currently two values are
# supported: 3.0 & 2.5.
# More precise (including per-session) tuning is possible via SQL and DPB.
#
# Per-database configurable.
#
# Type: string
#
#DataTypeCompatibility =
Just set 'DataTypeCompatibility
' to 2.5 or 3.0