In thunderbird, there is a problem when compacting the folder, the task fails. A new file is created (see http://kb.mozillazine.org/Nstmp_folders). This file is not deleted. Using large imap mailbox, normally after 3 months I have very more than 10 nstmp files.
It could be useful if CCcleaner could clean this enormous waste of space.
It is enough to delete all the nstmp*.* files in the same directory where the mailboxes are.
Thank a lot