Comment 122 for bug 376715

For all people bothering you need:

First (for minimizing to system tray at all):

https://addons.mozilla.org/en-us/thunderbird/addon/minimizetotray-revived/

Second (for minimizing to tray on startup):

https://addons.mozilla.org/de/thunderbird/addon/minimize-on-start-and-close/

Third (for starting thundebird with windows):

Under Win 10 use "Windows-Button + R" then type "shell:startup" then create a shotcut (e.g. to desktop) of Thunderbird and copy it in the previously opened startup folder

For other OS google how to add a program to startup.

And fast as this Thunderbird doesn't even need to be on your taskbar at all!

PS.: I know its not a support forum but after 14 years maybe it's nice someone clears it up, as beeing the most easy and intuitve thing a program has to offer (start).