Riscrivo la query (per me corretta)
SELECT
f.id as id,
f.forum_name as forum,
f.forum_desc as description,
count(forum_id) as threads,
u.name as mod
FROM forum_forum f, forum_users u
LEFT JOIN forum_posts p ON f.id = p.forum_id AND p.topic_id=0
LEFT JOIN forum_users u ON f.forum_moderator = u.id
GROUP BY f.id

Beh, fai una select da 2 tabelle e nel from non le elenchi entrambe?
Per me è quello!

HTH
Zappa