An example for text paragraphs: https://www.semanticscholar.org/paper/Reading-Online-Text%3A...
I think it really depends on individuals though. If you can memorise a dense screen of buttons you'll be able to work faster, avoid scrolling etc. But it'll make the UI harder to use for people who don't use it regularly.
Ultimately, every UI has to strike a balance. If you do it right you'll piss off both sides equally.
If you can use tiny fonts for everything else I'm sure the padding is less painful but it's super annoying if you have to scale things up and you can actually get completely lost in it if you have to use much fullscreen magnification.
This would be less frustrating if I could easily scale up UI fonts without also scaling up the whole UI proportionally, along with the padding.
In terms of apps in my life with annoying padding or wasteful use of screen real estate, though, I have to say Firefox doesn't even remotely make the list.
It, in fact, gives a specific practical disadvantage.
It's usually most important to quantify in the vertical axis; today's bloated touch-oriented UIs are horrific for 16:9 wide screens. Add up the taskbar, window title and tab bars, URL toolbar, the 72pt dickbar menu at the top of the web page with single-line labels, and the cookie banner at the bottom of the web page, and you're lucky to have half of the shortest dimension of your screen devoted to real content until you start excising the bad UI elements. It's like being back in the 1990s and seeing the old horrors of people who said yes to every adware toolbar that asked to install itself, except we're now wasting far more vertical space for far less functionality.
Furthermore, my feelings in this matter extend far beyond user interfaces: not only do I prefer clean user interfaces with generous use of negative space, I prefer that in my books, and the walls of my house, and the organization of my room. If my wall was covered in posters and sticky notes, instead of a nice clean beige with one or two posters, that would make me feel anxious as well, and it isn't because I don't know how to read a post-it.