In this query if you uncomment all 3 lines Firebird gives this errror:
"The cursor identified in the UPDATE or DELETE statement is not positioned on a row.
no current record for fetch operation."
But if you comment one or more of the lines - ORDER BY, the most outer coalesce or the first line the query runs without problems.
Under 2.1.3 there is no problem with the query.

SELECT
-- DEPOZIT.NUMBER || ' ' ||
-- COALESCE(
    IIF(NAST.BUL_ID IS NOT NULL, COALESCE(BULGARIANS.NAME,'') || ' ' || COALESCE(BULGARIANS.FAMILY,''), COALESCE(FOREIGNERS.NAME_CYR,FOREIGNERS.NAME_LAT))
-- ,'')
     AS NAME
FROM
    DEPOZIT LEFT JOIN NAST ON NAST.ID = DEPOZIT.NAST_ID
    LEFT JOIN BULGARIANS ON BULGARIANS.ID = NAST.BUL_ID
    LEFT JOIN FOREIGNERS ON FOREIGNERS.ID = NAST.FOR_ID
--ORDER BY 1