$ /opt/fb30/bin/isql -q
SQL> create database '192.168.0.220/3330:/var/db/fb30/tmp201405051238.fdb'; commit;
SQL> create table td(id int primary key using index td_pk, pid int, constraint td_fk foreign key(pid) references tm(id) using index td_fk);
SQL> commit;
SQL> show table td;
ID                              INTEGER Not Null
PID                             INTEGER Nullable
CONSTRAINT TD_FK:
  Foreign key (PID)    References TM (ID)
CONSTRAINT INTEG_4:
  Primary key (ID) uses explicit ascending index TD_PK

SQL> alter table td drop constraint td_fk;
Statement failed, SQLSTATE = 42000
unsuccessful metadata update
-ALTER TABLE TD failed
-DELETE operation is not allowed for system table RDB$REF_CONSTRAINTS
-At trigger 'RDB$TRIGGER_10'

SQL> show version;
ISQL Version: LI-T3.0.0.31099 Firebird 3.0 Alpha 2
Server version:
Firebird/Linux/AMD/Intel/x64 (access method), version "LI-T3.0.0.31099 Firebird 3.0 Alpha 2"
Firebird/Linux/AMD/Intel/x64 (remote server), version "LI-T3.0.0.31099 Firebird 3.0 Alpha 2/tcp (oel64)/P13"
Firebird/Linux/AMD/Intel/x64 (remote interface), version "LI-T3.0.0.31099 Firebird 3.0 Alpha 2/tcp (oel64)/P13"
on disk structure version 12.0