For most apps this is straightforward, as long as the app is running and showing on the taskbar, you simply right-click on the icon in the taskbar and a pin/unpin menu entry shows up.
However, lately, only "Write New Message" and "Open Address Book" and "Close Window" entries show up for Thunderbird. None of all the other methods and advice out there helps or works. Even manually creating a shortcut and copying it into
C:\Users\XXXX\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar%APPDATA%\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar
then restarting Explorer.exe (or even Windows itself) DOES NOT WORK.