Please upload an image showing exactly where this occurs.

I don't use the Thunderbird email function. It might be related to my default AOL, but I had 2 unread AOL messages when I took this photo. So still not sure what the little "1" represents?

1. GOTO Settings
2. Search for icon.
3. Click the button marked app icon and change the badge setting to what suits you.

Personally I turn it off because I always have more than 99 unread emails, and I do not care how many new messages I got in the last session.