How to Turn Off the Send Progress Dialog in Thunderbird

A hidden preference lets you disable the progress indicator when you send a message in Mozilla Thunderbird. 

To disable Mozilla Thunderbird's send progress dialog when it delivers an outgoing message:

1. Select Tools | Options... (or Thunderbird | Preferences...) from the menu.
2. Go to the Advanced tab.
3. Make sure the General category is open.
4. Click Config Editor....
5. Click I’ll be careful, I promise if prompted This might void your warranty!.
6. Type "show_send_progress" under Filter:.
7. Double-click mailnews.show_send_progress (under Preference Name to make sure false appears in the Value column.
8. Close the about:config configuration editor.
9. Click in the Thunderbird Preferences window.

To turn off the Send Progress dialog in Netscape or Mozilla SeaMonkey:

1. Open your user.js configuration file in any text editor and add the following line to it:
   1. user_pref("mailnews.show_send_progress", false);

That should get rid of the unnecessary send progress dialog. You can always turn it back on by changing false to true, of course.

(Updated October 2015, tested with Mozilla Thunderbird 38)