I need to solve a user problem that she wanted to recover deleted email's folders from the Trash folder in Thunderbird. Firstly, I found a very helpful link http://www.makeuseof.com/tag/how-to-recover-deleted-emails-in-thunderbird/ However, the method from that URL can only recover deleted emails only, not Folders.
And from the above method's inspiration, I found that the Thunderbird creates email folders in a pair files. That is the file of a "folder name" and the "folder name".msf file. For example, if you have a folder name called "test" under Trash folder (i.e. Trash.shd), you can find two files under the Trash.shd folder, "test" and "test.msf". So, the two files would store every emails in the test folders of Trash folder in Thunderbird.
Let's say: if the "test folder" is deteled in Thunderbird. The two files "test" and "test.msf" will be disappeared right away.
So, if you follow through what I am saying. You should understand that deleting a Thunderbird email folder means deleting the two files associated with the Thunderbird email folder name.
Therefore, to recover the deleted Thunderbird email folder means to recover the two files that were deleted by the Thunderbird when the folder were deleted inside the Thunderbird. So, you can use whatever file recovering tools (e.g. Recuva, undelete plus) to undelete the two files. Then, da da.... your email folder is coming back in seconds.
In case you don't know the folder location storing the Thudnerbird email:
For XP:
C:\Documents and Settings\YOUR USER NAME\Application Data\Thunderbird\Profiles\[random filename].default\Mail\Local Folders\
For Vista or Windows 7:
C:\Users\[YOUR USER NAME]\AppData\Roaming\Thunderbird\Profiles\[random filename].default\Mail\Local Folders\
Notes: Remember to close Thunderbird when you working on the files recovery.