Try rebuilding the icon cache:

https://www.howtogeek.com/232779/how-to-rebuild-a-broken-icon-cache-in-windows-10/

Thank you for your advice. In the meantime, a restart of Windows caused the generic "piece of paper" icon to disappear completely from the task bar (where Tbird had been pinned). So after launching Tbird the correct task bar icon appeared and I have pinned it. We'll see if it stays put.

I really have to ask...the frequent Windows 10 updates do not appear to corrupt the taskbar icons. It's disappointing that Mozilla cannot fix this. This time around after yet another forced Windows update, the Tbird taskbar icon remained but then upon a 2nd attempt to launch, Windows complained it can't find the "item."

I wonder why windows would not be able to find a program that was there only minutes before. Perhaps the issue is not Thunderbird but other software on your computer, perhaps aimed at protecting you like ccleaner or an internet security suite. I have lots of issues with my Thunderbird and windows in general, but I have lost a task bar pinned icon perhaps once (ever, going back probably 20 years.)