At least for me, as I have previous Outlook experience, following was missing in the default Thunderbird features:
- minimize to system tray on main window close
- show unread messages count in system tray
The first one, I used to solve with MinimizeToTray revived addon, however FireTray does for me both.
The only disadvantage for some I guess might be that FireTray seems to be supported on Linux only. However that's perfectly fine with my environment.