Server version:
Firebird/Windows/AMD/Intel/x64 (access method), version "WI-V4.0.0.2151 Firebird 4.0 Release Candidate 1"
on disk structure version 13.0
SQL>
SQL> SELECT CURRENT_TIMESTAMP, CAST('NOW' AS TIMESTAMP WITH TIME ZONE) FROM RDB$DATABASE;

                                        CURRENT_TIMESTAMP                                                      CAST
========================================================= =========================================================
2020-08-18 17:09:37.6420 Europe/Kiev                      2020-08-18 17:09:37.6420 Europe/Kiev

SQL> set time zone 'UTC';
SQL> SELECT CURRENT_TIMESTAMP, CAST('NOW' AS TIMESTAMP WITH TIME ZONE) FROM RDB$DATABASE;

                                        CURRENT_TIMESTAMP                                                      CAST
========================================================= =========================================================
2020-08-18 14:09:45.9640 UTC                              2020-08-18 17:09:45.9640 UTC