Firebird cast function does not support converting a numeric value to date, time or timestamp.
You can take advantage of the fact that Firebird supports arithmethic between dates and numeric values, so you can write your query like this:
select dateadd(second, ColAmountOfSeconds, cast('00:00:00' as time))
from myTable;
--or the equivalent:
select cast(cast('2013-01-01' as timestamp) + cast(ColAmountofSeconds as double precision) / 86400 as TIME)
from myTable;