Turn Off Mail Session Restore - Persisting Tabs At Startup
By default, Postbox remembers the set of folder tabs you had open when you last quit Postbox and it restores these tabs.
If you prefer to have Postbox start up without restoring any tabs:
- Open Config Editor.
- Type in mailnews.sessionstore.enabled into the "Filter" textbox.
- Double-click on that row for mailnews.sessionstore.enabled to toggle that value to false.