I have Thunderbird on three macs. All three are on Thunderbird version 78.6.1 using imap. One mac on Big Sur, the other two on Sierra. Interesting however, that only on one "Sierra" Mac, I cannot search: "email address=> search message. Neither pull-down menu "subject" or "contains" can be changed to other search criteria, and it does not accept any text in the search query. However, "search" from main windows ( top right ) works. On the other two macs everything is fine. Tried remove and re-install Thunderbird including profile, and configured new profile. Nothing changes, same problem. The only difference is, the working Thunderbird is on a macbook pro ( Sierra ), and the non functional is on a Mac-mini ( Sierra ). Any ideas ?