The most important aspect of email the the message content. Please modify space usage to maximize this area.
I cannot shrink the email list area to less than 8 messages. I could shrink this down to at least 3 in the prior version.
The header area is excessively tall with wide space between the lines. This has always bugged me.
The top field that contains the search bar seems unnecessary.
It appears that the message content is loaded before the header content. Very often it takes many seconds or tens of seconds for the header area to update to the newly loaded message especially if the new message has lots of images. I have experienced greater than a minute, sometimes.
I see I can turn off the menu bar but haven't yes as I'm still searching for solutions.
I did not have the header update time or the extended message list size on my last laptop (DEll Win10) with the prior version of TBird. I have restrained from installing the prior version so far but I'm tempted every time I use this new version.