A very simple add-on for Thunderbird 68.2+ that uses native messaging to display a persistent icon in the KDE systray.
You definitely need a native binary in addition to the add-on. There is only one at the moment for KDE: a Qt5 application written in C++.
I suggest using the latest Thunderbird version 68.3.1, which fixes a bug in an API function when searching for unread messages. It may or may not work with earlier versions.