Each development of Thunderbird means less features and plugins you are used to one. One the most basic features available on older versions of Thunderbird was the ability to close Thunderbird to the tray or minimize to tray rather than close down the software. Fortunately, I've found the development of a current addon called "Firetray" that you can use with Thunderbird to minimize to the tray: Source 5a.
We rest Minimize Thunderbird To Tray to share caption from way back when:
Today is: 19th June (GMT), in history on the 19th of June, 1954 AD the following event happened:
Betty Jameson wins LPGA Western Golf Open