[TECK]

You need to leave it:
ORDER BY thread.lastpost
because the table "forum" also contains also a "lastpost" row.

Also, you can just write it:
AND visible = '1'
no need to add the "thread.", no ambiguous clause there also.
Leave the code as I posted above, at the end with the 2 conditions:

	AND open = '1'
	AND visible = '1'
	ORDER BY thread.lastpost DESC
	LIMIT 5

WHERE and AND are conditions.