I received a message from someone in Firefox and answered it. I tagged their message. When I came back a little later, the tagged message had disappeared. Other messages with the same color tag were still visible, just that one. The person responded to my message, and Thunderbird said I had an unread message to that account, but I couldn't see it! I had to look at it on my phone, and answer it that way. My phone is set up to automatically send me back a copy of my own message. That message, also, was not visible in Thunderbird, but it said I had an unread message. I seem to have somehow accidentally hidden that thread, but I can't figure out what I did or how to unhide the thread. I thought Thunderbird was malfunctioning, and I closed the app and opened it again, but nothing changed. I'm on a Mac running OS X 10.15.6 Catalina. Thunderbird is 68.11.0 -- up to date.