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I have Ubuntu 18.10 and if I get a new E-mail from Thunderbird the red notifier on Dock (button) is not visible.
Have somebody an idea what happens?
In Thunderbird General preferences, check the following setting...
If you're using the Dash to Dock GNOME extension, the email message counter won't display, and this setting should be set to off.
Dash to Dock
Required, but never shown