Delete TRASH folder notification

[drc0304]

By, mistake, I checked the box on Delete all messages in TRASH to NOT show this message again. How do I get the "Delete all TRASH" dialog box back/

[morat]

Try toggling the mail.warn_on_delete_from_trash pref.

Similar thread
http://support.mozilla.org/en-US/questions/1155312

P.S.

You could also search for "warn" in the config editor. The bold entries are preferences with non-default values.