Using this Select:
SELECT DISTINCT t0_dep.ID
FROM NET_NET_DEVICE t1_nd, NET_NET_DEVICE t0_dep, NET_DEV_INTERCONNECTION t3_nd_dependantDevices_RELATION
WHERE ((t1_nd.ID = ?)) AND t1_nd.ID=t3_nd_dependantDevices_RELATION.PRIM_DEVID
AND t0_dep.ID=t3_nd_dependantDevices_RELATION.SECONDARY_DEVID

in FB 2.1.2 it produces this plan:
PLAN SORT (JOIN (T1_ND INDEX (PK_NET_NET_DEVICE), T3_ND_DEPENDANTDEVICES_RELATION INDEX (FK_NET_DEV_INTERCONNECTION_001), T0_DEP INDEX (PK_NET_NET_DEVICE)))

but In 2.5.RC1 it uses another plan:
PLAN SORT (JOIN (T0_DEP NATURAL, T3_ND_DEPENDANTDEVICES_RELATION INDEX (PK_NET_DEV_INTERCONNECTION), T1_ND INDEX (PK_NET_NET_DEVICE)))

As you can see selecting T0_DEP natural is used instead of PK_NET_NET_DEVICE

The tables definitions are below:

CREATE TABLE NET_NET_DEVICE
(
  ID INTEGER NOT NULL,
 CONSTRAINT PK_NET_NET_DEVICE PRIMARY KEY (ID)
);

CREATE TABLE NET_DEV_INTERCONNECTION
(
  PRIM_DEVID INTEGER NOT NULL,
  SECONDARY_DEVID INTEGER NOT NULL,
  INTERCONNECT_LEVEL INTEGER,
 CONSTRAINT PK_NET_DEV_INTERCONNECTION PRIMARY KEY (PRIM_DEVID, SECONDARY_DEVID)
);

ALTER TABLE NET_DEV_INTERCONNECTION ADD CONSTRAINT FK_NET_DEV_INTERCONNECTION_001
  FOREIGN KEY (PRIM_DEVID) REFERENCES NET_NET_DEVICE
  (ID);

ALTER TABLE NET_DEV_INTERCONNECTION ADD CONSTRAINT FK_NET_DEV_INTERCONNECTION_002
  FOREIGN KEY (SECONDARY_DEVID) REFERENCES NET_NET_DEVICE
  (ID);