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”