Rated 5 out of 5 stars

by OkeN on Sept. 20, 2019 ·

With Thunderbird 68 just use RBTray.
It's a small Windows open source program that runs in the background and allows almost any window to be minimized to the system tray by right clicking its minimize button.

Tried it and works perfectly and only takes 2 mb memory.

https://github.com/benbuck/rbtray