Chapter 7 of the Language Reference manual lists all the systems tables and their fields. It is under the section called "System Tables and Views".
I had never really looked at these tables very closely but I was able to throw together the following query that showed me the tables I had created and the columns in those tables. Is this the type of query you're looking for? If it is, make sure you test it before you use it. I just glanced at the results to make sure they were "correct" but didn't spend much time analyzing them.
SELECT a.RDB$RELATION_NAME, b.RDB$FIELD_NAME, b.RDB$FIELD_ID, d.RDB$TYPE_NAME,
FROM RDB$RELATIONS a
INNER JOIN RDB$RELATION_FIELDS b
ON a.RDB$RELATION_NAME = b.RDB$RELATION_NAME
INNER JOIN RDB$FIELDS c
ON b.RDB$FIELD_SOURCE = c.RDB$FIELD_NAME
INNER JOIN RDB$TYPES d
ON c.RDB$FIELD_TYPE = d.RDB$TYPE
WHERE a.RDB$SYSTEM_FLAG = 0
AND d.RDB$FIELD_NAME = 'RDB$FIELD_TYPE'
ORDER BY a.RDB$RELATION_NAME, b.RDB$FIELD_ID