You must be joking. The Supernova UI redesign is an unmitigated disaster. They unnecessarily butchered the look and feel of Thunderbird to the point where people are switching to forks.
Well I like Supernova better, it made me switch to Thunderbird from em Client. (I always wanted to switch to Thunderbird, but until Supernova I never liked the UX enough to keep using it)
The previous UI made no sense. Having the email viewer on the bottom and the list on the top instead of side by side makes absolutely no sense when most emails are designed for viewing in portrait.
Give me the 1990s GUI back.