[Eikinskjaldi]

It will require the creation of a temporary table, be a big slow process and quite possibly crash your server. You would be better off scripting it in php or any other language that can connect to your db.

This query will return a list of firstpostid, user's posts:

[sql] select concat_ws(',',t.firstpostid, group_concat(p.postid))
from thread t
join post p using (threadid)
where p.userid=your_users_id
group by t.firstpostid[/sql]

example output

 37916,75977,75978,79764,79767,81094,81096

You can then take that list and pass it into a second query
[sql]select p.username, p.pagetext from post where postid in (above_list)[/sql]

Note that group_concat has a character limit, which can be changed if you have the appropriate mysqld permissions.