Okay one criticism: the UI density "dense" is too dense and squeezes the text. So I use the normal mode.
Meanwhile, unlike KDE Plasma 6, Thunderbird added more unneeded barriers.
Also, the message "bubbles" are not cut off at the bottom limit where the content ends, but have a random space below left. This means you need to scroll more.
The star instead of the yellow dot for "new" wastes space.
I'll be honest, and forgive me I don't have a horse in the desktop race, but the newer is much better. It's cleaner with logical lines and margins that mean it doesn't feel cluttered.
