IMAP Trash folder
From MozillaZine Knowledge Base
Thunderbird assumes that your Trash folder is called "Trash" and not some other name, such as "Deleted Items". This can create a problem if you have a IMAP account and the IMAP server uses a different name for the Trash folder. Thunderbird would create its own Trash folder (named "Trash") while your mail server would use the other folder (e.g., "Deleted Items"), and you might see both folders in the Thunderbird folders pane.
You can rename the Trash folder using a custom-strings.txt file in the optional Chrome directory in the programs installation directory. That file is mainly meant to localize text in the user interface but it can be used to change hard-coded folder names. For example if you wanted to rename the Trash folder to rubbish:
Localized versions of Thunderbird sometimes have different names for some of the hard-coded folders. For example the EN-GB (UK) version uses "Deleted" for the Trash folder.
If you want to use "Gelöscht" (German for Trash) you might have to enter it as "Gel&APY-scht", since the German Umlaute has to be encoded differently. See Creating a escaped version of a non-ASCII folder name .
An alternative solution is to use the FolderFlags add-on to flag a folder as the Trash folder. That should work for both IMAP and POP accounts though not for a global inbox (Local Folders).
To specify a different Trash folder for an IMAP account in Thunderbird:
1. Completely exit Thunderbird if it is running.
3. Open your "prefs.js" file with a text editor such as Notepad to find the server# used to represent your IMAP account. It could be "server2", "server5", "server7", or some other number depending on your own individual account setup. Look for a line similar to this, corresponding to your own IMAP acccount:
4. Insert a line like the following into "prefs.js", with the correct server# and correct name for the Trash folder:
user_pref("mail.server.server#.trash_folder_name", "<correct Trash name>");
user_pref("mail.server.server2.trash_folder_name", "Deleted Items");
If you want to use "Gelöscht" (German for Trash) you would have to write it in the user_pref as "Gel&APY-scht", since the German Umlaute has to be encoded differently.
5. Restart Thunderbird. You should now see the new Trash folder (e.g., "Deleted Items") showing the trash can icon. (If not, exit and restart Thunderbird.) If the old Trash folder is still showing, you should now be able to delete it. If you can't delete it through Thunderbird, then delete it through the account's webmail interface.
6. If you deleted the Trash folder in the web front end of your mail provider, you may need to cancel the IMAP folder subscriptions, too. It may take a while for the subscription changes to take effect.
Thunderbird typically relies upon the "Allow server to override these name spaces" setting to specify the IMAP server directory in Tools -> Account Settings -> Server Settings -> Advanced. If these instructions don't work and that field is empty try setting it to "mail". If that doesn't help exit Thunderbird, delete the "Trash." and "Trash.msf" files for that account, delete the panacea.dat file (Mail folder cache) in the root of the profile, and then see if it uses the new trash folder.  
Creating a escaped version of a non-ASCII folder name
$ python -c 'print repr("Gelöschte Objekte")' # (German version of Exchange) 'Gel\xf6schte Objekte'
$ python -c 'print repr("Gelöscht")' # imap.1and1.de 'Gel\xf6scht'