Thunderbird add-on that adds blink feature to the new-mail alert.
https://addons.thunderbird.net/en-US/thunderbird/addon/blinking-alert/
The default new-mail alert is subtle if the alert sound was turned off.
This extension adds blink feature to the new-mail alert.
You can also disable auto-hiding of the alert.
After the extension is installed, some options are added to the "Customize New Mail Alert" dialog.
Open "Options" dialog, select "General" page, and click "Customize" button right to the "Show an alert" check box.
If auto-hiding was disabled, you have to click close[X] button of the alert box to close it.