When I start Thunderbird, I get a standard icon* in the task bar, but then sometimes the icon changes. The icon changes when a child window is opened. It produces two icons, both related to the child window.
It seems to me to be a bug that the icon does not change back when the child window is closed; or even that different icons are not present for the two windows.
It is not obvious how to get the first icon back, but opening some other child windows (e.g. Help, Search) achieves this.
Here are some icons I’ve noticed. Until you learn to recognise these, Thunderbird can easily be ‘lost’.
Write a new message 
A reminder from the calendar 
Write a new event or a new task 
The only thing I have found is this post (in German) https://forums.linuxmint.com/viewtopic.php?p=2548762&hilit=thunderbird+icon#p2548762 which acknowledges the problem, but has no fix.
Is there any way of controlling this behaviour?
I have Thunderbird 128.4.2esr with Linux Mint 22 and Cinnamon. It could be that the problem originated with version 128.
*The starting icon can be changed in file /usr/share/applications/thunderbird.desktop, the line: Icon=thunderbird
The name seems to refer to a file under /usr/share/icons/ .
Changing this does not affect icons for child windows.
