Make Spacebar Advance to the Next Unread Message in the Next Folder

Currently when reading a message, the space bar will advance to the next unread message in that folder. If you would like the spacebar to go to the next folder with unread messages do the following: 

  1. Open Config Editor.
  2. Type in mailnews.nav_crosses_folders into the "Filter" textbox.
  3. Double-click on that row for mailnews.nav_crosses_folders.
  4. Change the value to 0.
  5. Restart Postbox