My "Inbox" folder started showing "(1)" meaning 1 unread email in the folder, even when there are none unread. The num will go up but never goes below 1.

My Thunderbird Inbox always shows "Inbox (1)" even when there are no unread emails. If I receive new emails the number unread will always show one more than there are actually present. Then when they've all been read it goes back to still showing 1 unread. Is there a way to fix this? Thanks.