I ran the install to update to 78.2.1. At the end it said I had to reboot to finish the update. I did. When I opened Thunderbird after rebooting it again said I had to reboot to finish installation. I did...multiple times with the same results. I also tried uninstalling Thunderbird but it also told me I had to reboot to finish the update. I found the Installation log. I think it indicates a proper install. At the end of the log: "Setting as default mail application"--"Reboot Required to finish Installation"--"Mozilla Thunderbird Installation Finished 2020-09-06 11:59:23." It appears both "thunderbird.exe.moz-delete" and "thunderbird.exe.moz-upgrade" are still in the "Mozilla Thunderbird" program directory. I would think that, on restart, the former should be deleted and the latter should be renamed taking off the ".moz-upgrade. In case it makes a difference, there are two users on the computer and it appears that they both use the same profile (one at a time). I don't know what else to tell you. Ask if you need more info. Thanks a bunch!