Turn Off the Send Progress Dialog in Mozilla Thunderbird

To disable Mozilla Thunderbird's send progress dialog when it delivers an outgoing message:
Select Tools | Options... (or Thunderbird | Preferences...) from the menu.
Go to the Advanced tab.
Make sure the General category is open.
Click Config Editor....
Type "show_send_progress" under Filter:.
Double-click mailnews.show_send_progress (under Preference Name to make sure false appears in the Value column.
Close the configuration editor and the options dialog.