What do you find “old-fashioned” about Thunderbird? Do you not consider an interface “new” if they don’t change it and hide all the common features every five minutes like Microsoft does? It’s an email client, you read your emails in it. How would you do it better?
Heh I just ran into the invisible icons issue recently, for whatever reason I am no longer able to accept Teams meeting. Yeah that’s definitely a shitty thing. But more whitespace? In other words, less visible information on the screen which requires more scrolling or clicking to other screens? Sorry, that just sounds annoying and less productive.