Mostly we find these incessant update requests do not originate in Thunderbird. It is a new feature of some antivirus programs to prompt for software updates. I have no idea how they do that, what I do know is they are quite prone to getting in wrong. The small number that are actually requiring an update and for some reason not getting the update actually installed are also cause mostly caused by antivirus programs. The update tries to install new files and the bumbling antivirus prevents that with it's scanning. If you are seeing a trend in my answer you would be correct. Antivirus is probably the greatest cause of failure in Thunderbird doing it's job. It is also the application that routinely asks you to stick a great hole in your encrypted communications so it can scan.