well first you would look in account settings and see if Thunderbird is setup to delete messages from the trash. These settings are in the server settings part of the account settings. Then you might look at repairing the folder to see if the index is bad Right click the folder and select properties and then repair. That would normally be followed by a look for other software that might be removing mail. anti virus and so called "optimiser" applications have a long history of causing inadvertent loss of mail from deleted folders.