One specific email does not show in TB, but is present in IMAP

I received an IMAP email as normal, from an email address that I often write with. What I always do: the IMAP inbox is forwarded move on arrival to the local inbox. It had an attachment, and I downloaded that as normal. Then after 5 minutes I wanted to start work on the reply to the message in the local inbox and ... the message was gone. Nowhere to be found. Not in the IMAP inbox (of course), not in junk, not in trash. I looked in the on-line IMAP account, and there the message was present as normal. Then I searched for the email and there it shows: in "Inbox". And I see one strange thing that I never saw: column Message threads says: "This subthread is ignored". So that is the problem. Probably I hit accidentally some key (k ?) and this caused the "kill". Now how to un-ignore it? There is this Help page: https://support.mozilla.org/en-US/kb/ignore-threads and it says: "To un-ignore a thread that has been marked ignored, select a message in the thread, right-click on it and again select Ignore Thread. ". But right-clicking the email (in the search results) shows NO such option. What can I do? Thanks!