I have multiple accounts, all via IMAP and each account has some subfolders. I use the "Unread Folders" view/filter to keep track of unread emails. That works great most of the time, however it keeps happening that I miss unread emails because the shown folders do not update. When there is no unread email in the folder at the time I launch Thunderbird (or when I activate "Unread Folders") and then during the same session new emails arrive for that folder, the folder does not get added to the unread folders list. And therefore the emails get overlooked.