SysTray-X: A system tray extension for Thunderbird. Needs both the addon AND the companion app installed to work. Will not work with TB flatpaks or snaps.
I installed systray-x in Firefox 75 for fun......Amazed it installed with some warnings. But more amazed that the preferences menu is just working. WTF are they doing at Thunderbird ???
Preferences are not shown in the preference dialog. Preferences can be changed with the systemtray icon (except for the account)