To load Thunderbird at Startup, you need to paste Thunderbird shortcut into this folder. Open this in Window Explorer and follow below to find the folder.
C:\Users\[user login name]\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup

To start Thunderbird during windows startup for any user who logins, simple open this folder and copy Thunderbird shortcut into this folder.
C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup

Thunderbird_notification_startup

Next

Go to Mozilla addons and download https://addons.mozilla.org/en-us/thunderbird/addon/minimizetotray-revived/

Next

Open your Thunderbird client and install the above addon.

Go to Tools–> Addons—>[Beside search box] click on the icon to select “Install Add ons from File”—> choose your file and install—>Restart

Thunderbird_notification_startup01

Next

Windows 7 – On your right corner of your monitor, you can see little icons on your taskbar,

Thunderbird_notification_startup02

click on the arrow “Customize”, select “show icon and notification for Thunderbird”.

Thunderbird_notification_startup03

After you finish settings all the configurations, restart your Thunderbird. Click on “minimize” on your window not “close”. You should be able to view notifications of new email alert.