Thunderbird has limited setting for emptying the trash. The only one I am really aware of is to empty the trash on exit. Otherwise it does nothing with trash. However, many mail provider routinely empty trash is a scheduled basis like weekly or daily. What your provider imposes you would have to ask them.

As most folk use IMAP accounts, server trash functions will over ride any settings in Thunderbird.

Emptying Trash will produce a confirmation window if the preference mailnews.emptyTrash.dontAskAgain is false, the default, in Preferences/General/Indexing, Config. editor.

To empty the trash folder, simply open the trash folder and highlight the first trashed message. Then, holding the shift key down, hold the page down key until you've highlighted all the trash you wish to get rid of. Then press the delete key and a prompt will come up asking if you want to permanently delete all these messages. Click on OK and voila, it's history. I just did this with my trash folder about 3 minutes ago.