I am afraid that I am not at all happy with the new version of Thunderbird. It may be that as I get used to it I will find some of the new features helpful, but right now I am more concerned about what was taken away that was helpful:
a) I keep getting 'not responding' -- especially when trying to set my theme 'yet again (and again, and again)!!!! I am mostly shut-in due to illness and the themes make a difference emotionally compared to a stark 'black and white'. Furthermore, what is the point of having TB themes when they don't hold -- I often have to re-set mine several times a day
b) tabs are now hidden -- I need to colour-code messages so I know which I need to get back to soon or can wait. Having them hidden under that new right arrow makes extra steps to do so
c) the row of custom colours no longer hold: what is the point of being able set a row of custom colour if they don't hold!!! I used different colours to indicate different responses in a thread -- thereby having an immediate way to see the order of responses.
d) I need layers of sub-folders -- often to add a new name to a group I belong to, or a new year. I have been asking again and again over several years for the ability to create a new sub-folder WHILE one is moving an email from the inbox to a folder or sub-folder. Having to 'go' to the folder first, make a new sub-subfolder, return to the inbox, and then move the email is an unnecessary waste of time.
e) 'a picture says a thousand words' -- emojs are so useful in this way, especially when one is tired or can't think of the right words to say. This feature was taken away years ago, and I wish that TB would bring it back. The minimal 'yellow faces' are minimal useful.
I am not techy enough to really understand what you have written below - what is a 'message context menu' and 'mock-ups'. However, it seems to me that the TB functons (at least the ones that I need) are getting less and less useful and increasing 'cognitive burden'.
Sorry to be so negative, but I do wonder who TB is trying to please/appease -- it is not this user.