For gmail IMAP, uncheck 'Place a copy in', since gmail automatically copies sent messages to Sent Mail, which should be subscribed.

I did try that too by uncheck "Place a copy in" and sent out a test email. Again, the email was in the sent folder briefly and then automatically moved to the trash folder. Is this an issue with gmail or thunderfire? Any other suggestions would be greatly appreciated.

That doesn't happen on my setup. 'Place a copy in' is unchecked, Sent is not subscribed, and Sent Mail is subscribed. Do you have any filters applied? A Google search indicates this is a common problem, with many remedies.

No, I do not have any filters applied

Check folder retention policy

• Right click on the subscribed gmail 'Sent Mail' folder and select 'Properties'
• Select 'Retention Policy' tab
• Usually the 'Use my account settings' option is selected.

What have you got selected ?

But then you need to check your account settings. 'Account Settings' select 'Synchronisation & Storage' for the gmail account

What is selected in that section ? Have you selected 'Keep messages in all folders.....' ? If you click on 'Advanced..' button - is the 'Sent Mail' folder selected ? Under 'Disc Space' : Is 'Synchronise all messages locally regardless of age' selected or something else ? Is 'Don't delete any messages' selected or something else?

Do you have any other means of accessing those emails, such as another laptop or maybe a phone, OR maybe even another email client on your computer which is operating in the background (I did come across someone who discovered they had Outlook still getting emails and it was causing a problem - they thought they had switched it off, but it was in the Startup list and unknowingly operating in the background) - because if you do then you need to check those devices and other email clients because if any of them delete a sent mail, it will get get deleted on server and then when Thunderbird imap Sent Mail folder synchronises with server it will also get updated to show what is on server.