Until recently, whenever I opened a program by clicking on the program icon pinned to the taskbar, that icon was replaced by the active program button. This is the behavior I want in order to keep the taskbar uncluttered. However, I recently mistakenly unpinned an icon for Mozilla Thunderbird and after re-pinning the program start icon, now the active program button appears in addition to the startup icon rather than replacing it. I have pinned and unpinned the program several times, and even after a restart of the machine, the same result persists. The problem seems to be limited to Thunderbird, at least in my limited research.
If I pin other new programs to the taskbar, the expected replacement behavior occurs, even after un-pinning and re-pinning, e.g for Microsoft Word.
If I un-pin and re-pin programs that have been pinned to the taskbar from before the time this issue occurred with Thunderbird, the expected replacement behavior occurs, e.g Mozilla Firefox