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. Go to the Advanced settings panel.
    (Mac) Preferences > Advanced > General
    (Windows) Options > Advanced > General
  2. Click on the Config Editor button.
  3. Click I'll be careful, I promise!
  4. In the Filter textbox, type in mailnews.nav_crosses_folders
  5. Double-click on that preference, and change the value to 0.
  6. Restart Postbox