If you have the same issue as myself, then you are seeing Thunderbird open twice every time you click the icon to open your email. Then a pop up saying that the folder is being processed / populated. Then you try to close one of the two Thunderbird windows, and they both close on you.
There are two solutions to stop Thunderbird from opening so many times. (sometimes it’s more than just two instances of Thunderbird).
1.Fix for Thunderbird . This one just makes it so you can use your email client.
Open Thunderbird.
Close the pop up dialogue.
Maximize both instances of Thunderbird.
Put mouse over taskbar and right click one instance of Thunderbird.
select “close”
And that should to it for you to use Thunderbird for now. Problem is, the next time you select “restore down” option on Thunderbird and then close without Maximizing Thunderbird, it will continue to happen.
2.The fix for Thunderbird
Simply just minimize all instances of Thunderbird, to the taskbar. right click each instance of Thunderbird and close them. All except for one. Now maximize that one. Now close that one. Now open Thunderbird again and the problem should be resolved.