The following query appears to put Firebird 1.5.3 into a loop.
Processor goes to 100% and the query never appears to finish.

SELECT A.LD_LOCATION, A.LD_LOAD_NUMBER, A.LD_LOAD_SUFFIX, A.LD_LH_MILES,
E.LD_LOCATION AS LOAD_LOC, E.LD_LOAD_NUMBER AS LOAD_NUM,
E.LD_LOAD_SUFFIX AS LOAD_SFX, E.LD_NUM_STOPS, E.LD_TTL_REVENUE,
E.LD_TTL_ASS_REVENUE, E.LD_TTL_CART_AMOUNT
FROM LOADS A
INNER JOIN LD_STOP B ON B.LDS_TEMPLATE_FLAG = 'N' AND
B.LDS_LOCATION = A.LD_LOCATION AND
B.LDS_LOAD_NUMBER = A.LD_LOAD_NUMBER AND
B.LDS_LOAD_SFX = A.LD_LOAD_SUFFIX
INNER JOIN MF_STOP_STOPS C ON C.MFSS_STOP_ID = B.LDS_STOP_ID
INNER JOIN LD_STOP D ON D.LDS_STOP_ID = C.MFSS_LDS_STOP_ID
INNER JOIN LOADS E ON E.LD_TEMPLATE_FLAG = 'N' AND
E.LD_LOCATION = D.LDS_LOCATION AND
E.LD_LOAD_NUMBER = D.LDS_LOAD_NUMBER AND
E.LD_LOAD_SUFFIX = D.LDS_LOAD_SFX
WHERE A.LD_TEMPLATE_FLAG = 'N' AND
A.LD_MANIFEST_FLAG <> ' '
ORDER BY A.LD_TEMPLATE_FLAG, A.LD_LOCATION, A.LD_LOAD_NUMBER,
A.LD_LOAD_SUFFIX

The plan shows that indices are being used for all JOINS:

PLAN SORT (JOIN (B INDEX (LDS_LOAD_KEY),A INDEX (LD_LOAD_NUMBER_KEY),E
INDEX (LD_LOAD_NUMBER_KEY),C INDEX (RDB$PRIMARY87),D INDEX
(RDB$PRIMARY39)))

The same query with the last JOIN (LOADS E) omitted completes in less
than a minute. I have let the full query run for 15 minutes, with
Firebird at 100% CPU the whole time, before I finally killed it.

This is the query that works without problems. As I said before it
runs in less that 1 minute with Firebird at 5% - 10% of CPU.

SELECT A.LD_LOCATION, A.LD_LOAD_NUMBER, A.LD_LOAD_SUFFIX, A.LD_LH_MILES,
D.LDS_LOCATION AS LOAD_LOC, D.LDS_LOAD_NUMBER AS LOAD_NUM,
D.LDS_LOAD_SFX AS LOAD_SFX
FROM LOADS A
INNER JOIN LD_STOP B ON B.LDS_TEMPLATE_FLAG = 'N' AND
B.LDS_LOCATION = A.LD_LOCATION AND
B.LDS_LOAD_NUMBER = A.LD_LOAD_NUMBER AND
B.LDS_LOAD_SFX = A.LD_LOAD_SUFFIX
INNER JOIN MF_STOP_STOPS C ON C.MFSS_STOP_ID = B.LDS_STOP_ID
INNER JOIN LD_STOP D ON D.LDS_STOP_ID = C.MFSS_LDS_STOP_ID
WHERE A.LD_TEMPLATE_FLAG = 'N' AND
A.LD_MANIFEST_FLAG <> ' '
ORDER BY A.LD_TEMPLATE_FLAG, A.LD_LOCATION, A.LD_LOAD_NUMBER,
A.LD_LOAD_SUFFIX

Is this a bug or is there some way to restructure the query to make it
run faster.