It may not be gone forever. It depends on how Avast deleted it, and how well it deleted it.
All of your Emails are stored in your Profile Folder in Thunderbird, even the ones deleted from Trash.
Click on the Help tab in Thunderbird, then click on Troubleshooting Information.
Under Application Basics, look for Profile Folder, and click on show folder.
You should see your Mail folder, open it and click on the Trash file, not the Trash Folder, and open it in Word Pad.
This file could be huge, depending on how long you have been using Thunderbird, and could take a long time to load.
If you do find the Email, it will have full headers. Scroll to the top of the header, and change the number after X-Mozilla-Status to 0.
Save the file. When you close it, you will be asked if you want to save it. Click Yes.
The Email should appear in your Inbox.
Attachment Below: