Screens these days are huge and high resolution, and my eyes aren't getting any younger. I'm finding that I like more white space and padding as time goes on.

For normal Thunderbird, I swapped from the more compact options to the most loose/padded options.

Screens are huge, that's why I want to take advantage of it and fit more windows on them, not less. But with how foamy modern apps are, it can be a struggle to have two windows side by side on a 2560px wide 27" screen and not have content cut off that would've been perfectly visible on an 800px screen 20 years ago.

For your eyes you'd better have larger text instead of wasting the same space with floating

I do both.

But the design we're discussing doesn't

There's a lot of padding in the screenshot, but you can also see that any user can reduce it by resizing the sidebar and inbox column.

This wouldn't help with big left padding, nor with the top one

Much padding, such space. Wow.

(Couldn't resist...)