I am using Ubuntu 18.04 and had the Dock with pinned applications on the left hand side of my monitor. Thunderbird is one of those pinned applications. When new email arrived the icon would display a red bubble with an indication of the number of unread emails. Recently I changed my monitor setup and as a result I moved the Dock from the left hand to the bottom side of the monitor. I now notice that this red bubble does not appear any longer. Each time when I want to check for new email I have to open Thunderbird and look at the inbox. Having to do this multiple times per day is inconvenient.
How can I get this "unread email" notification working again?