OK, try this procedure. First, backup your Profile (
http://kb.mozillazine.org/Profile_backup )
Create a new sub-folder under Local Folders. Copy all of the messages currently in your Trash folder to that new sub-folder.
Confirm that you have made the recommended backup of your Profile.
Close Thunderbird and navigate to your Profile folder. Delete both the file Trash.msf and the file Trash with no extension.
Restart Thunderbird and delete a message, which should cause Thunderbird to create a new Trash folder. Copy any messages you MUST have in your Trash folder from the sub-folder created above to the Trash folder. Compact the Trash folder after those messages are copied in.
The "Real Fix" entry in this article --
http://kb.mozillazine.org/Compacting_folders#Real_fix also describes this process.