Try this:
SELECT
cat.category_id
, cat.category
, cat.stickypost
, t.category_id
, t.topic_id
, c.created_on
FROM
forum_comments AS c
INNER JOIN forum_topics AS t USING ( topic_id )
INNER JOIN forum_category AS cat USING ( category_id )
INNER JOIN
(
SELECT
t.category_id
, MAX(c.created_on) AS created_on
FROM
forum_comments AS c
INNER JOIN forum_topics AS t USING ( topic_id )
WHERE c.confirmed = '1' AND t.confirmed = '1'
GROUP BY t.category_id
) AS c2 ON ( c2.category_id = t.category_id AND c2.created_on = c.created_on )