Hello Eddie,
You can recover your deleted emails by manual methods. Follow the below mention steps:
1. Login into Mozilla Thunderbird, here is the demo mail is used to demonstrate how to retrieve deleted Mozilla email. I have deleted the demo mail from Inbox and it’s in Trash folder.

2. Now I have deleted the demo mail from Trash folder as well.
3. Go to the default location of Thunderbird mail client to start Run by pressing Win + R on keyboard and type %APPDATA%.

You can also find location by following options > Account Settings > Server Settings > Local Directory

4. By the above step, you will find the location, then open any folder (Inbox, Draft, Trash, Sent) with Notepad++.

5. By going down you will found X-Mozilla-Status:(some number) which contains all the deleted emails which you have removed from Inbox or Trash folder. Edit the number 0009 to 0. Save and close Notepad++.

6. Open Thunderbird and Refresh it. Now you can view same mail in Trash folder.

Hope this information will help you!
Thanks