Are they visible when you log into webmail? If so, see if those folders are subscribed:

- on the main mail screen in the L. column, R. click the account (just above the Inbox) > Subscribe

Yes, but they come from three different email accounts and of course only the recent ones, some of the emails stored in TB on my hard drive go back years (decades?) and are long gone from any webmail account. So it's really a local issue. I went into the mail folder on my hard drive (Users/Username/Library/Thunderbird/Profiles/xyz.default-release/Mail) and the missing folder was there alright, but I didn't have permissions ("no entry" sign). Very peculiar. I checked the permissions and it said I had read/write (even stranger), so I changed it to read only then back to read/write and it still showed the no entry symbol. So I gave read/write permissions to everyone and that solved the permissions problem. I checked the folder again, and the email file had disappeared. The msf file was there, but not the email.

I have no idea how that happened but I got a backup from Time Machine and put it back. All well now. But a mystery nonetheless.