User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:72.0) Gecko/20100101 Firefox/72.0
Steps to reproduce:
Without resorting to now-abandoned add-ons or third-party applications, it's impossible to send Thunderbird to the tray area. A program such as Thunderbird, that will usually be open on the user's desktop for some time, pinging for new mail every X minutes, should really have this functionality in-built.
The only add-on that worked for this is now abandoned and unusable, thanks to Thunderbird disabling older add-ons, as well as the code of the add-on in question being outdated to the point that an entire rewrite is required (one that the add-on author has said he has no intention of doing any time soon).
This should be an optional in-built feature. It's frankly bizarre that it's been requested for the last 17 years (!) but still hasn't been deemed worth adding.
Minimizing/closing to the system tray is what should happen, assuming the user has enabled that option in the program's preferences.