thank you, I had not found that function. I imagined the function that I wanted than queried. First I found the function in Thunderbird 3 beta, there it is called 'Smart Folders', that may do something else, similar.

"Smart Folders
The folder pane offers a Smart Folders mode which combines special mailboxes, like Inbox, from multiple accounts. Smart Folders is now on by default. "

http://www.mozillamessaging.com/en-US/thunderbird/3.0b4/releasenotes/#whatsnew

no use beta for production.