Currently Thunderbird supports system tray on Windows - just close the software and an icon will be presented on Windows system tray.
Unfortunately this feature is missing on Linux. In past there were several plugins to do that, like several versions of Minimize-to-tray, however the plugin technology for Thunderbird (and Firefox) has changed over time, and these plugins are no longer supported.
Currently if I want to use this feature on Linux I need install an additional software like systray-x or KDocker, which it is weird.
So, please turn this feature native for Linux too!