I can't specifically say this is a beta 3 issue but only started happening after installing NVDA 2020-4 beta 3.
I am using the latest version of Win 10 and Thunderbird. After using Thunderbird I use alt f4 to close the program. It appears that thunderbird closes as it should. I have thunderbird set to announce new mail. The strange thing is that with Thunderbird not open I still get announcements of new Email which tells me that Thunderbird was not closed properly when using alt f4 to close it. Thunderbird is not showing in the system tray or on the task bar when these announcements are announced and I must open Thunderbird by using the icon on the desktop.