There is no function to scan all folders of an IMAP account for new emails. However, with IMAP any Folder can contain new eMails. Here for example I use procmail which is a little superior to mozmails filters to sort my mail. However, Mozilla only looks in the mbox for new mail thus I have to search through the whole folder tree by hand to find my eMails.
*** This bug has been marked as a duplicate of 18266 ***