The first Thunderbird 115 beta is now available, so if you're already running beta, please update.

This Thunderbird Beta includes the first "Supernova" UI features, which provide an updated user interface, designed to be more modern, customizable, and with code that is easier to maintain and improve.

Release notes:

https://www.thunderbird.net/en-US/thunderbird/115.0beta/releasenotes/