I completely solved the problem by going back and re-installing Thunderbird 128.11.1esr on my MacBook Pro running Catalina 10.15.7. No more overheating, no more continuous fan running. Thunderbird 140.0.1esr needs a fix.
I noticed exactly the same behavior on my MacBook Pro (16-inch, 2019) running Catalina 10.15.7 and Thunderbird 140.0.1esr . The fans run continuously. Thunderbird uses 5% of CPU with nothing else but Finder active.
