Please bear with me as I'm not a power user, so (A) I may not use the right terminology and (B) I skipped a few versions so I'm not exactly sure when this behavior changed. Mac OS 10.12.5, Thunderbird 52.1.1 (64 bit) In the folder pane (the list of your email folders), folders that contain new/unseen messages get highlighter - in blue in my case. The way this used to work before my last update (which has been from 4x.x.x - I don't remember my last version number, it may have been 46) was different: a folder with new messages in it would get highlighted, but once I opened the folder, the highlight would go away, indicating the now the folder with the new messages has been opened so there are no more "unseen" messages in that folder. I'm using the word "unseen", because the highlight doesn't indicate unread messages, but folder with new messages since that folder was last opened (at least that's my observation of how it worked). In this new version, this has changed slightly: when I open a folder with unseen messages, the highlight stays on, which is confusing. A scenario where this is counterintuitive is when I have a folder open, so all messages in it have been "seen", and then a new message arrives while that folder is still open. In the old version, the highlight would turn off as soon as I opened the folder, and if a new message arrived while the folder was still open, the highlight would come back, indicating that there is a new message in the folder. In the new version, the highlight (if the folder was highlighted when I opened it) the highlight doesn't go away until I click away from the folder (open another folder).