How to Get New Message Alerts in Thunderbird for Windows

Step1: Select the three lines at the top right to open the menu, then select Settings.

Step2: With the General tab selected on the left, scroll down to the Incoming Mails section. Then Select the box next to Show an alert so it’s enabled.
And If you’d like, select Customize to edit some additional options, then choose OK to save.

This lets you configure the alert’s contents and display duration, letting you see the sender, subject, and message preview in the alert.

Credit : https://www.lifewire.com/new-email-alerts-thunderbird-1173081