You've deleted an email and emptied the trash and you just realized you need that email back! In Thunderbird as long as you didn't compact the folders you can un-delete that email.
This only works if your account in Thunderbird is setup as a POP3.
Under Hidden files and folders section, select "Show hidden files, folders, and drives" and then click the "OK" button.
Then navigate to your Thunderbird's profile folder. Here are the paths that it can be located:
Windows 7/Vista/XP/2000: %AppData%\Thunderbird\Profiles\
From there you'll need to navigate to your email profile folder. The folder could be named xxxxxxxx.default where xxxxxxxx would be a random string of letters and numbers
If your Thunderbird handles more than one email account you'll have more than one xxxxxxxx.default folder. Once you've located your .default folder open it and then click on the Mail folder. In the "Mail" folder there could might be two folders, one called "Local Folders" and another where the name could vary.
Example of my folder path:
You'll be looking for a file called Inbox -- not Inbox.msf. Once the Inbox file is located, open it with notepad.
When the Inbox file is opened in notepad. Scroll thru or do a search on a key -word that would be in the email your looking for. Once it's found, look in the headers near the top for X-Mozilla-Status, next to it will be the numbers 0009. Delete the "0009" and replace it with 0 ( Zero ).
Save the file and close it out. Open your Thunderbird and you'll see that the email is now in the inbox folder.