Probably open the task manager (Alt+Ctrl+Delete) and select from the menu. Then close the running instance, it may be shown either as a program or a background process.

To begin with, *WHY* should I have to close a running instance???? This is a function that should work against a running Thunderbird instance. And why would I use (Alt+Ctrl+Delete) to kill a task on a Linux system?

I just switched to Betterbird, and THAT is working correctly.

So this problem is *****STILL***** occurring with Thunderbird 128.4.0esr & Firefox 132.0 (Fedora 40).

It is DEFINITELY a bug in Thunderbird, as if I set Betterbird as my email client (to handle mailto links) it works with NO problem.