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.

Thunderbird 78 also finally comes with its own tray icon, but it only works when minimizing the window, not when closing it. TBTray 0.10 used to provide its own tray icon, but TBTray 2.0 instead makes use of Thunderbird's own tray icon, and merely intercepts the requests to close the window and turns them into requests to minimize the window instead.

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.

Note: Requires Microsoft Visual C++ 2015-2019 Redistributable, with same bitness (x86, x64) as your Thunderbird.

0. Make sure that you are using Thunderbird 78 or later.
1. Download the latest TBTray release.
2. Extract the archive anywhere you want, %localappdata%\TBTray would be a good place for instance. I would not recommend to put it in the same folder as Thunderbird, although it should be possible in theory.
3. Figure out whether you run a 32-bit or 64-bit version of Thunderbird. If you are not sure, check whether Thunderbird is installed in Program Files (64-bit on a 64-bit system, 32-bit on a 32-bit system) or Program Files (x86) (32-bit on a 64-bit system).
4. Launch the TBTray executable in the folder that matches your Thunderbird bitness, i.e. if you run a 32-bit Thunderbird, then run TBTray.exe in the 32-bit folder of TBTray.
5. To automatically start TBTray on Windows startup, run register.cmd in the folder that matches your Thunderbird bitness, i.e. if you run a 32-bit Thunderbird, then run register.cmd in the 32-bit folder of TBTray. If you ever decide to move the executable to a different location, you will need to run register.cmd again.
6. To completely uninstall, run unregister.cmd and delete the extracted files.

TBTray intercepts some window messages sent to Thunderbird, rejecting window close events and instead minimizing the window, causing Thunderbird to make use of its 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 through the config editor should solve that problem.

Alternatively, Alt + F4 still works intended - you can still keep using that shortcut to send Thunderbird to the notification area if you are used it.

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.

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.