If that account has always had that problem.

Access the Profile folder mail account and delete the Trash files.

• Profile folders are hidden files, so you need to make hidden files visible first.
• Then close Thunderbird.
• Locate Profile folder

Linux and Unix Profile folders are located here:

• ~/.thunderbird/<Profile name>/

However, if you're using a third party build from Debian or Ubuntu, those builds store your profile folder here:

• ~/.mozilla-thunderbird<Profile name>.

Both are hidden folders.

To show hidden files in Nautilus (Gnome desktop's default file browser), choose "View -> Show Hidden Files".

• click on 'Mail' folder
• click on mail account name
• look for and delete the 'Trash' file and the 'Trash.msf' file
• Restart Thunderbird and a new Delete folder will be created.

or try If the Deleted file is full, right click on Deleted folder and select 'Empty' and then right click and select 'compact'. Otherwise do the first option, but you will lose all in the deleted file.

That did it! Thanks Toad-Hall!

Sidenote for others with this problem: I had tried shutting Thunderbird, deleting Trash folder, recreating Trash folder myself, then restarted Thunderbird and had the same condition.  :-( When I did as Todd-Hall suggested and let Thunderbird create the Trash Folder, all was back to normal.  :-)