Downgrading is not supported. That us because the data structures in the profile folder are changed by the update process and there is no backward compatibility. You can use the -allow-downgrade command line option I discuss here https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html but you are almost certain to see something not make the transition. A better result will be obtained by reinstalling 115 and fixing whatever the issue is. Personally I do not like the user interface, but to all intents and purposes it does appear to be well received and stable.