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?

“needs more whitespace and rollover disclosure on invisible icons”

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.

Information density MUST be suitable for humans. Usability and productivity both have nothing in common with amount of clicking and scrolling required.

Just imagine making your font size something about 5px. And 1.0 as a line height. Sounds good, isn’t it? There ia so much information displayed on the screen.

If you need less information on screen, they invented a great thing for that in the 90s: resizable windows. And later, HiDPI-aware interface scaling.

That is the right way to control information density. The user can control both of these however they like and set it to whatever they work best with, and it applies across the system. You can’t do that with usually custom written interfaces that insist on putting like two lines of text worth of whitespace between every UI element.