<blockquote> How do I move these to the new version of Thunderbird I am being prompted to install? </blockquote> You don't. Profile files (i.e. your data) are stored separately from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem. Having said that, it's always recommended to have a recent backup of your Thunderbird profile folder, so go ahead and create one. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile After that upgrade Thunderbird when prompted.