See the comments here about KDocker:

https://addons.thunderbird.net/en-us/thunderbird/addon/minimize-on-close/

Thanks for the recommendation. I successfully installed it with `sudo apt install kdocker` as it is in the Ubuntu repositories.

However this is not very straightforward to have to look for and click on a extern program to be able to do that.

I could use a keyboard shorcut but all the other apps for which it makes sense to close to tray already have a "close to tray" option (Hexchat, Transmission, Discord etc.) and it feels a bit hacky to have a shortcut just for Thunderbird.

I don't understand the logic behind removing this option. Anyway thank you.

Minimize to tray is built into TB for Windows, and since that covers about 95% of users, I guess the developers have decided the effort to make it available in Linux is not worth the effort.

Actually it is not so bad. Once you've docked an app with Kdocker, it keeps the app docked so after after you've reopened it, you can just click the icon tray to close to tray again. The problem is you can't use kdocker to start Thunderbird in a .desktop file if you are using a primary password. The password prompt will make Kdocker timeout.

I found a much better solution if you are using Linux. It's called Birdtray. It's available on Flathub

Only caveat is, on Linux Mint 20.3 at least, you must use the flathub Thunderbird version in order to use it and I did not my where I had to copy my profile directory to migrate to the flathub version.

If you install another version of TB, you can make it open an existing profile: Help/More Troubleshooting, about:profiles, create a new profile, name it, Choose folder..., Browse to the existing profile folder.