'Menu icon (3 lines) > options > Options > 'General' tab select 'Show a Tray icon'

This will show an envelope in the bottom right Task Bar. There may be an icon in bottom right Task Bar to show/hide icons. So if the option to 'Show a Tray icon' was already selected then maybe it is currently one of those icons that are hidden.

To make the tray icon always visible, you need to open Windows Settings and set TB to 'On' in the notification area settings. On W10, Settings/Personalization/Taskbar/Notification area/Select which icons appear in the taskbar.

A better method is provided by this add-on:

https://addons.thunderbird.net/en-US/thunderbird/addon/unread-badge/

Can't open the file.

To Toad-Hall: Thanks, but that's not what I asked. There's no notification not showing. I'd like something in the Taskbar that says there's a new email.

Funny that the phone apps are often better than PC apps.

ral52 said

Can't open the file.

Right-click the Download button on the web page, Save link as..., download the xpi file. In TB Tools/Add-ons/Extensions, click the gear icon next to search, Install add-on from file..., select the xpi file. Version 2 of the add-on is for TB 68, v. 1.2 for TB 60.

https://addons.thunderbird.net/en-US/thunderbird/addon/unread-badge/versions/