In Thunderbird v184.108.40.206, I have accidentally activated a feature that groups messages in my Inbox by day; I thought I was typing in Word, but Thunderbird was the active application, so some combination of letters typed turned the feature on. Now I get a bold header of "Old mail," under which is listed mail from before yesterday; then a bold header of "Yesterday," with yesterday's messages, then a bold header of "Today," with today's messages. Each header has a +/- box next to it, so the messages grouped under that header can be displayed or hidden.
I don't want the headers, but I'm not sure how I got them, so I can't figure out how to turn them off. I don't see any obvious menu options to do this. Any ideas?