Textual words from them:
It’s our first step towards a more modern, more beautiful, and more customizable Thunderbird experience. We think you’re going to love it, and we are endlessly grateful for all of your support throughout the years 💙
Eh… it’s pretty, I guess, but it’s just too “appified” for power users. My Thunderbird UI is festooned with useful buttons and menus for quick access, and I like the old style of having elements tightly spaced to maximize contextual awareness.
This reeks of form over function. Yuck.