Problem
When you have configured Thunderbird to check for new Emails everey few minutes, only the status of your inbox gets updated. Mailfolders which contain pre-filtered Mails (e.g. by procmail) wont update their status.
Solution
Change the Thunderbird internal behavior with the following steps:
In Thunderbird select Edit / Preferences / Advanced / General / Config Editor
- Confirm that you will be carefull.
Type mail.check_all_imap_folders_for_new into the filter line.
Double-click the setting so that the Value changes to true