After the x-th reincarnation of the MinimizeToTray add-on for Thunderbird broke in Thunderbird 68, it seems like it becomes more and more difficult, if not impossible, to solve the issue at hand - keeping Thunderbird minimized in the notification area when closing or minimizing it - using Web Extensions.
I know that BirdTray exists, and it's even cross-platform. However, it tries to solve way more problems than I have and uses Qt (no offense, I really like the framework), so it's not quite as light-weight as I think a background process should be.
So I decided to fork a program a friend of mine wrote - traktouch, as it solves a very similar problem. I could have written it from scratch, but this way I didn't have to write most of the boilerplate code.
TBTray intercepts some window messages sent to Thunderbird, rejecting window minimize and close events and instead hiding the window and creating a tray icon.
To do this, TBTray checks for the presence of the Thunderbird main window, and if it finds the window, injects a library into the Thunderbird process to hook into the message queue. TBTray keeps running in the background, in case you want to restart Thunderbird at some point.
The most likely cause is that you have mail.tabs.drawInTitlebar set to true
(which is the default value) - setting it to false should solve that problem.
Note: If you want to get this fixed, consider submitting a pull request - I do not have the time required to debug and fix a feature I am not using.
Through the File menu, or the context menu of the tray icon.
This program does not come with any options, because it is just supposed to fix exactly the problem I had. Please fork the repository if you want the program to behave differently.
To automatically start TBTray on Windows startup, run register.cmd.
To uninstall, run unregister.cmd.