Rated 5 out of 5 stars

Splendid little add-on, rsjtdrjgfuzkfg, but would it be possible for you to add that little feature to MoC, right-mouse-click on the icon in the tray to close Thunderbird?

This review is for a previous version of the add-on (2.0.1.1).  This user has a previous review of this add-on.

Assuming you mean the built-in icon available on Windows only: No. That icon and its menu cannot be modified by add-ons, it exists purely in native code. You might be able use a separate application to minimize Thunderbird to the tray instead (instead of using the built-in feature).