Recursive relationship on a many to many table

I'm not familiar with Firebird, but this works in SQL Server, so is hopefully similar / enough to get you on track:

WITH TEST (IDRoot, IDPARENT, IDCHILD) AS
(

  SELECT P0.IDPROD, C0.IDParent, C0.IDCHILD
  FROM PROD AS P0
  left outer join COMP C0 on C0.IDParent = P0.IDPROD
  WHERE P0.IDProd not in (select IDChild from COMP)

  UNION ALL

  SELECT T.IDRoot, C1.IDPARENT, C1.IDCHILD
  FROM COMP AS C1
  inner join TEST AS T on T.IDCHILD = C1.IDPARENT

)
SELECT * FROM TEST

Hope that helps.

SQL Fiddle Version: http://sqlfiddle.com/#!6/22f84/7

Notes

Includ a column to denote the root of the tree as well as parent/child - since there may be multiple trees if we're not specifying a particular root:

WITH TEST (IDRoot, IDPARENT, IDCHILD) AS

Treat any product which is not a child as a ROOT (i.e. first item in a tree).

WHERE P0.IDProd not in (select IDChild from COMP)

EDIT: Answer to comments

Query on any node to see all of its relatives:

The simple way to filter on any node would be to amend the above statement's WHERE P0.IDProd not in (select IDChild from COMP) with WHERE P0.IDProd = IdImInterestedIn. However if you want to use the CTE for a view, then run queries over this static query you could use the code below - you can then filter on IDProd (select * from test where IDProd = IdImInterestedIn) to see that item's ancestors and descendants.

WITH TEST (IDProd, IDRelation, Generation) AS
(

  SELECT IDPROD
  , IDPROD
  , 0
  FROM PROD 

  UNION ALL

  SELECT T.IDPROD
  , C.IdParent 
  , T.Generation - 1
  FROM TEST AS T
  inner join Comp as C 
  on C.IdChild = T.IDRelation
  where t.Generation <= 0

  UNION ALL

  SELECT T.IDPROD
  , C.IdChild 
  , T.Generation + 1
  FROM TEST AS T
  inner join Comp as C 
  on C.IdParent = T.IDRelation
  where t.Generation >= 0

)
SELECT * 
FROM TEST
order by IDProd, Generation

SQL Fiddle: http://sqlfiddle.com/#!6/22f84/15

See a root node's full tree in a single column

WITH TEST (IDRoot, IDPARENT, IDCHILD, TREE) AS
(

  SELECT P0.IDPROD, C0.IDParent, C0.IDCHILD, cast(P0.IDPROD as nvarchar(max)) + coalesce(', ' + cast(C0.IDCHILD as nvarchar(max)),'')
  FROM PROD AS P0
  left outer join COMP C0 on C0.IDParent = P0.IDPROD
  WHERE P0.IDProd not in (select IDChild from COMP)

  UNION ALL

  SELECT T.IDRoot, C1.IDPARENT, C1.IDCHILD, TREE + coalesce(', ' + cast(C1.IDCHILD as nvarchar(max)),'')
  FROM COMP AS C1
  inner join TEST AS T on T.IDCHILD = C1.IDPARENT

)
SELECT * 
FROM TEST 
order by IDRoot

SQL Fiddle: http://sqlfiddle.com/#!6/22f84/19

EDIT: Answer to Additional Comments

with cte (tree_root_no, tree_row_no, relation_sort, relation_chart, Name, id, avoid_circular_ref) as
(
       select row_number() over (order by p.idprod) 
       , 1 
       , cast(row_number() over (order by p.idprod) as nvarchar(max)) 
       , cast('-' as nvarchar(max)) 
       , p.NAME 
       , p.IDPROD 
       , ',' + cast(p.IDPROD as nvarchar(max)) + ',' 
       from PROD p
       where p.IDPROD not in (select IDCHILD from COMP) --if it's nothing's child, it's a tree's root

       union all

       select cte.tree_root_no
       , cte.tree_row_no + 1
       , cte.relation_sort + cast(row_number() over (order by p.idprod) as nvarchar(max)) 
       , replace(relation_chart,'-','|') + ' -' 
       , p.NAME
       , p.IDPROD
       , cte.avoid_circular_ref + cast(p.IDPROD as nvarchar(max)) + ','
       from cte
       inner join COMP c on c.IDPARENT = cte.id
       inner join PROD p on p.IDPROD = c.IDCHILD
       where charindex(',' + cast(p.IDPROD as nvarchar(max)) + ',', cte.avoid_circular_ref) = 0
)
select tree_root_no, tree_row_no, relation_sort, relation_chart, id, name 
from cte 
order by tree_root_no, relation_sort

SQL Fiddle: http://sqlfiddle.com/#!6/4397f/9

Update to show each path

This one's a nasty hack, but the only way I could think of to solve your puzzle; this gives each path through the trees its own number:

;with inner_cte (parent, child, sequence, treePath) as (

    select null
    , p.IDPROD
    , 1
    , ',' + CAST(p.idprod as nvarchar(max)) + ','
    from @prod p
    where IDPROD not in 
    (
        select IDCHILD from @comp
    )

    union all

    select cte.child
    , c.IDCHILD
    , cte.sequence + 1
    , cte.treePath + CAST(c.IDCHILD as nvarchar(max)) + ','
    from inner_cte cte
    inner join @comp c on c.IDPARENT = cte.child

)
, outer_cte (id, value, pathNo, sequence, parent, treePath) as
(
    select icte.child, p.NAME, ROW_NUMBER() over (order by icte.child), icte.sequence, icte.parent, icte.treePath
    from inner_cte icte
    inner join @prod p on p.IDPROD = icte.child
    where icte.child not in (select coalesce(icte2.parent,-1) from inner_cte icte2) 

    union all

    select icte.child, p.NAME, octe.pathNo,icte.sequence, icte.parent, icte.treePath
    from outer_cte octe
    inner join inner_cte icte on icte.child = octe.parent and CHARINDEX(icte.treePath, octe.treePath) > 0
    inner join @prod p on p.IDPROD = icte.child

)
select id, value, pathNo
from outer_cte
order by pathNo, sequence

SQL Fiddle here: http://sqlfiddle.com/#!6/5a16e/1