Is your email account POP or IMAP? POP accounts only connect to the Inbox. With IMAP all mail and folders stay on the server and you just view them remotely with an email client.

If it is IMAP have you subscribed to the Sent folder? File-Subscribe

Thanks for working to help me with this!

They are both IMAP accounts. I am subscribed to the 'sent' folder. I tried also subscribing to a folder called 'sent messages' and this added a separate 'sent messages' folder to Thunderbird which did have the messages sent through mac mail (icloud) - but didn't include the messages sent from Thunderbird, which are in the 'sent' folder.

Any other recommendations?

From the Menu Bar select Tools-Account Settings-Copies and Folders Change the storage location for Thunderbird to the other folder.

Thanks. I tried that but nothing changed.

I noticed in mac mail (icloud) folders - there is a section called 'mailboxes' with inbox, drafts, sent, junk, trash. The sent messages in this sent folder are only the ones sent from mac mail, not from Thunderbird.

Below that there are folders under the heading '.mac' - the folders here are the folders I've created - which are synced with Thunderbird. There is, however, a folder in this list called 'sent' and it has all the emails that were sent through Thunderbird but not from mac mail.

Thanks again for your help.