

The O365 support people here of course believe the issue is with Thunderbird, a setting of some kind.

This worked for me too, but it's less than ideal, partly because Outlook comes with a lot of junk folders I don't want and can't delete, mostly because after 20+ good years with Thunderbird I've accumulated a lot of folders that I want to keep but don't need to see every day, for which "unsubscribe" used to be the perfect solution, leaving the folder on the server in case I needed it but getting it off my desk, so to speak.

The only solution anyone seems to have found is a workaround, not a fix: Server Settings/Advanced/ uncheck "Show only subscribed folders" will restore all folders. It look like other people have experienced versions of this issue before, especially with Office365: e.g. If I subscribe to all my folders, then close and re-open Thunderbird, I see the folders for a split second and then they disappear, leaving me with only Inbox and Trash. If I right-click my account and select "subscribe," I can see all my folders, and I can check the boxes and select subscribe, but when I click "ok" it doesn't save the checkmark(s). About a week ago, all my folders disappeared from Thunderbird except Inbox and Trash. I'm using the latest version of Thunderbird (78.13.0) to access email in my company's Office365 account.
