This 'sounds like' the thunderbird prefs.js file is being prevented from being updated in the profile. Checking the profile to verify that this just one such file and not several (such as prefs-1.js, prefs-2.js, etc) may help remove that as a possibility. That sometimes happens if an antivirus or firewall or other software is intercepting the closing of thunderbird.

Well gosh, david, you may have nailed it (or maybe not). Last night, after I received your reply, I sent myself a message (click "Send," it went), and contrived a draft to no-one and saved it (Cmd-s, Cmd-w). Then I quit TB (now 137) and relaunched it, and the Sent folder re-appeared, first containing only that one sent message, and then rapidly filling up with the other sent messages from the past. The drafts folder then also re-appeared. There was at first a strange yellow star on that folder icon, then it would open after re-clicking, with the new draft and the earlier ones (and the star disappeared). Still no Templates folder. Then I quit TB first (so no other software would intercept the closing) and after that shut down the MBP.

But rebooting this morning, TB lunched with the Sent, Drafts, and Templates folders missing, so I repeated the drill above, with the same results.

As far as I know, I am just using the default profile, never having created any new one. When I look for Profile Folder in "Troubleshooting Information" and Show in Finder, I see just one Profile (j09yot5j.default) with just one "prefs.js" file. I can open that in BBEdit, but then I am lost in the maze...

Please help me take this to the next level...

Thanks, A.

It seems to me that your account on the MBP is an IMAP account. Next time when folders are disappearing in your account do a R-click on the account name in the folder pane and choose "Subscribe...". In the upcoming folder list (image) check (select) the folders you want to display.

Why do you rely on Local Folders?

Thanks, Mapenzi, this seems to have done the trick, even if it took two runs through the procedure to do that. Maybe some of the earlier efforts contributed, but I don't know enough to say. The situation now seems stable.

And I rely on Local Folders because I have 3.3 GB of accumulated archives over the last 40 years, starting with Eudora before I moved to TB. That was all POP then, and with IMAP I thought that would be unworkable on the mail servers and a burden on the bandwidth. So I store them on my own computer.

This way I am vulnerable to glitches on my own computer instead of relying on an external system of unknown stability and reliability. One such glitch seems to have happened recently, which Repair Folder under R-click to Properties has failed to correct. Oh well, the lost mail was from the mid-Nineties, and I think I can live with the loss.

Thanks for your feedback. I advise you to make regular backups with Time Machine, or at least backups on an external disk of the "Thunderbird" folder in your user-library.

Right on. I maintain regular Time Machine backups on an external flash drive, and also a bootable flash-drive backup with Super Duper. These are habits originating in a time when systems, while simpler, were less reliable than now. But failures can still happen, and redundancy always gives us an edge.

I also recognize that as things have gotten more complicated, their potential for instability (as I have seen with TB) increases exponentially, so we all have to be extra alert and hedge our bets.

Thanks again. I really appreciate the knowledge you guys have, and your sharing of it.

A.

Just a comment here: If not done already, I encourage doing a test recovery with your backup. I say that because I routinely see people posting, wanting help because their backup software cannot restore their thunderbird profile.

We are in "great minds think alike" mode here. Just recently, migrating from a 2013 iMac to a hand-me-down 2019 MBP, I did just that using a Time Machine backup. It worked well. And I say "well" instead of "perfectly" because there were strange configuration glitches within TB that we have been addressing in this thread and earlier. And none of those glitches may have been because of the migration; we simply can't tell.

Lots of ambiguity from potential decay of intensively-used older SSDs, despite passing "Disk Repair" several times.

But all the Local Folder folders came over (apparently) perfectly, and what data corruption I have seen later in one folder may have happened for some entirely different reason. Again, we can't tell.

And don't need to, even if it might be nice or even useful. Onward...