The folder pane redesign was covered in depth in this blog post:
https://blog.thunderbird.net/2023/02/thunderbird-115-supernova-preview-the-new-folder-pane/
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".
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