The folder pane redesign was covered in depth in this blog post:
blog.thunderbird.net/2023/02/t…

As you can see from my screenshot, I removed that big blue "New Message" button, because I can just use Ctrl+N.

Thank you @thunderbird for retaining just enough customizability for power users while making the UI more appealing to the "touchscreen generation".

Thunderbird 115 Supernova Preview: The New Folder Pane

Thunderbird 115 introduces a brand new Folder Pane to make webmail users feel right at home. But what about veteran users? Find out inside!

^{Jason Evangelho (The Thunderbird Blog)}