Close and re-open Thunderbird. All of my IMAP accounts disappeared as soon as I updated from 91 to 102, too. I was pretty shocked and annoyed. I figured I was going to have to restore a backup, but first I tried closing and re-opening Thunderbird. It proceeded to re-index all of the IMAP accounts. I lost all of the folder settings (column selections, widths, and order), but I don't appear to have lost any emails.

iCloud uses IMAP, so my guess is that you're seeing something similar.

I had the same problem too.

I run 4 email accounts through Thunderbird, and two remained entirely unaffected. I only had problems with my iCloud and Fastmail accounts.

I upgraded to 1.02.3.2 for Mac, and my iCloud and also Fastmail accounts disappeared. Stranger still, where my iCloud account had been in the left hand pane of email accounts and subfolders of those accounts there was just a blank folder with a folder symbol (like a foolscap paper folder) rather than the envelope email symbol for the folder associated with the other accounts. It was blank in name and when you clicked on it gave you the set up page "Welcome to Mozilla Thunderbird" title at the top, the version number, and then "Choose what to set up", Import from another programme" etc options.

The iCloud account had been a 'primary' account in the sense that it was the first one on the accounts list, and the first to appear in the left hand pane.

The Fastmail account, on the other hand, was there in name only, by which I mean it had a folder with the email address of the account as the folder title, but no Inbox and subfolders, and the same set up page as above, just with the email account name at the top of the set up page.

I checked all my server settings for both accounts, as the accounts still existed in Thunderbird despite not displaying in the left hand pane, and found that they were all correct and as they had been in the previous "Nineties" version of Thunderbird (e.g. 93.x.x) rather than this "Hundreds" one (e.g. 102.x.x), including the smpt "mail.messagingengine.com" . There was one alternative smtp address I could use (imap.fastmail.com), so I changed that and it worked, displaying my Fastmail account with all folders etc at the bottom of the left hand pane series of email accounts, but had put "IMAP" after the email address in the title of the folder (e.g xxx@xxxaccount.com (IMAP)".

The old Fastmail folder still sat in its original position, but with no folders etc.

I upgraded from 102.3.2 to 102.4.1 to see if that would make a difference, and it did. Whether this was the new version or just a restart I can't say. However, the iCloud account is now back up and running in the left hand pane, and I now have 2 Fastmail accounts in the left hand pane, the original and the (IMAP) titled one, but they are showing folders in a different folder, and the old one shows no deleted folder.

I don't know if this is related, but before the upgrade/ restart in the account settings under the "Copies and Folders" section of each account none of the folders you could normally choose in the drop down menu for sent, archived, drafts and templates were available, and it said something to the effect that they were unavailable. After the upgrade/ restart, all of the drop down folder options have been restored.

"Upgraded" to 102.5 and now I have no server settings, no outgoing or incoming.

Local settings and email are still in settings.

I was (am?) using OWL for outlook integration.

This is not good.

PC windows 10.