Do you have Norton Password Manager affecting TB passwords? Normally, a password can be changed by removing it from Saved Passwords, restarting TB, entering the new password in the prompt and checking the box to have it stored in Saved Passwords. This is for accounts with 'normal password' authentication, and is a bit different with other types of authentication. If there are no entries in Saved Passwords, it often points to an external app that is managing passwords.

Thanks, sfhowes. I do not have Norton Password Manager installed, and have (temporarily) removed Norton 360. I do use Bitwarden as a password manager, but it is set up more as a reference rather than to autofill or manage passwords. There are no passwords in "Saved Passwords", and when I get the password entry prompt at login, there is no box to choose to save the password. I wonder if this is a result of an older preference setting? I used Seamonkey for years and moved to Thunderbird a few years ago.

If you've used the same profile over several versions of TB, there might be some obsolete file that is affecting the passwords. I recall there was an issue with secmod.db.

If you can't find the source of the problem, try adding an IMAP account to a new profile and see if it works correctly. Help/Troubleshooting Info, about:profiles, to create and manage profiles. The current profile is unaffected, and if the new one works, data can be transferred from the old to the new.

sfhowes, we have liftoff! Deleting secmod.db (actually renaming it...) did allow passwords to be saved. However I also first needed to create a user.js file containing just the line:

   user_pref("signon.rememberSignons", true);

(with a few comments). I had also renamed logins.json, key3.db, and key4.db, but these deletions did not appear to make any difference. Thunderbird did NOT create new key3.db or secmod.db files, but did create new key4.db and login.json files. Thanks very much indeed for your help!

To reset all passwords, the usual advice is to delete or rename logins.json, key4.db and cert9.db, all of which are recreated automatically when TB restarts. key3.db is left over from old versions. It shouldn't be necessary to create a user.js file, as the preference signon.rememberSignons is accessible in Settings/General, Config. editor (default is true).

This topic further illustrates why I always create a new profile for the major, annual releases, despite TB people asserting that this is unnecessary.