What is currently on beta is a major refresh with some big changes and is still in developement, so it is being finished and improved. As it is now, usable amount of search text is not available at the first glance. Message list in vertical view mode is quite nicely done but message filter box need to be expended a little.Currently messages body have very little screen space in classic layout. We need a quick option to shrink and expand message header view in classic layout. It is my personal opinion but currently is no different then previous UI.Message Header area can be a little darker.To me, it doesn't feel separate enough from main message body.Compact mode without visible separators in message list unlike old UI feels a little congested to me. By default UI density was compact for me instead of 'Default'.In dark theme, top bar and tab bar doesn't have that nice shadow line separating them on lower brightness settings. UI now is very clean and nice but I have one issue with dark theme.Reducing its height so that lower edge is just in middle of Mail and Address-book icons in Spaces toolbar will make it look much better. With only global search bar in it that is high enough for text to appear in it, it looks especially jarring and out of place. the height of Top Bar is a little to long.I don't have release version installed on my device.

I have done a clean install of Thunderbird beta.

Mozilla thunderbird