Test:
====
recreate table test(
  id int
  ,pid int
  ,constraint test_id_pk primary key(id) using descending index test_id_desc_pk
  ,constraint test_fk_pid foreign key(pid) references test(id) using descending index test_pid_desc_fk
);
commit;
show table test;
show index;


-- this is output of SHOW TABLE command:
ID INTEGER Not Null
PID INTEGER Nullable
CONSTRAINT TEST_FK_PID:
  Foreign key (PID) References TEST (ID) ----------------------------------- [ 1 ]
CONSTRAINT TEST_ID_PK:
  Primary key (ID) uses explicit descending index TEST_ID_DESC_PK

-- this is output of SHOW INDEX command:
TEST_ID_DESC_PK UNIQUE DESCENDING INDEX ON TEST(ID)
TEST_PID_DESC_FK DESCENDING INDEX ON TEST(PID)

(similar in 2.1, 2.5 & 3.0)

Name of index used in FK: TEST_PID_DESC_FK -- occures only in the SHOW INDEX output.
No index name between "Foreign key (PID)" and "References TEST (ID)" but it was explicitly specified in the table DDL (see marked line "[ 1 ]").

ISQL -X result:
===========
SET SQL DIALECT 3;

/* CREATE DATABASE . . .; */
COMMIT WORK;

/* Table: TEST, Owner: SYSDBA */
CREATE TABLE TEST (ID INTEGER NOT NULL,
        PID INTEGER,
CONSTRAINT TEST_ID_PK PRIMARY KEY (ID) USING DESCENDING INDEX TEST_ID_DESC_PK);

ALTER TABLE TEST ADD CONSTRAINT TEST_FK_PID FOREIGN KEY (PID) REFERENCES TEST (ID);

-- here also no index specified in ALTER TABLE ADD <FK> statement.

Result will be not good if such database metadata is restored from script, than making data migration and start to analyze performance etc - no correlation between "standard" index name like "RDB$*****" and its table.