I have been using Thunderbird for a few years and I have seen it evolve. The latest beta release 112.0b1 (64-bit) is certainly a step backward. The previous release was awesome. It has a UI that was professional looking and the navigation was easy. Here are some of my observations -
1. My favorite folders are now at the bottom, not sure who made the decision to move the favorites at the bottom. The whole objective of a favorite is to put it in front of your eyes.
2. The view pane and the email list pane do not seem balanced. The new UI is very unbalanced. The real estate to view your email has been shortened.
3. Why did the global search appears on the top? seems very weird. In the previous release, it was nicely synced with the entire UI.
4. Why is my email account folder all expanded? I have to scroll down to see all my emails which are more clicks.
I have done a clean install of Thunderbird beta. I don't have release version installed on my device. Overall new UI is quite good. Here are some of my observations.
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. Please check the release notes https://www.thunderbird.net/notes/beta/ - there is much information there, including why this release is different and will be incomplete for a few more weeks to complete the development and improvements. Plus there are pointers to a beta forum.
1. The order of folder types is determined by the order they were enabled. If you uncheck the other folder types to the point where only favorites is listed, then you favorites will be at the top and you can enable the other types.
2. "The view pane and the email list pane do not seem balanced." is difficult to understand without a screen shot. But I suspect you are using Vertical View?
3. "Why did the global search appears on the top?" This is an area that is not yet complete. It is now a unified toolbar. Please see https://thunderbird.topicbox.com/groups/beta/T99ff74f89b697778/unified-toolbar-description-and-statu...
4. Hard to understand without a screen shot.
Hopefully some things are better with beta 113.
Hope this helps.