I have the following SQL (PostgreSQL) query:
SELECT ff.*, fp.*
FROM fibra ff, fibra fp
JOIN cables cp ON fp.cable_id = cp.id
LEFT OUTER JOIN terceiro  ced_p         
        Convert all the joins in your query to be explicit to avoid the issue you're having--don't leave some implicit and others explicit.
This should work:
SELECT ff.*, fp.*
  FROM fibra ff
  JOIN fibra fp ON ff.fibra_pai_id = fp.id 
  JOIN cables cp ON fp.cable_id = cp.id
  LEFT OUTER JOIN terceiro  ced_pai ON ced_pai.id = cp.cedente_id
  LEFT OUTER JOIN terceiro tp ON tp.id = fp.terceiro_id
  JOIN cables cf ON ff.cable_id = cf.id
  LEFT OUTER JOIN terceiro ced_f ON ced_f.id = cf.cedente_id
  LEFT OUTER JOIN terceiro tf ON tf.id = ff.terceiro_id
WHERE
 ff.cable_id IN (8,9,10) 
 AND fp.cable_id IN (8,9,10)
                                                                        You are mixing implicit and explicit JOINs. That's generally confusing to read, and leads to unexpected order-of-evaluation problems, as you've just discovered.
You should consistently use JOIN ... ON syntax everywhere; avoid the legacy FROM table1, table2. If you correct your query to use an explicit JOIN instead of FROM fibra ff, fibra fp, eg FROM fibra ff INNER JOIN fibra fp ON (ff.fibra_pai_id = fp.id) and omit ff.fibra_pai_id = fp.id from the WHERE clause, you should get the expected result.
See this question that A.H. linked to:
Mixing explicit and implicit joins fails with "There is an entry for table ... but it cannot be referenced from this part of the query"