They do/might but adding all that empty space in the desktop version of Firefox is quite pointless, considering that it has a mobile version.

I get that there are people with Surfaces and whatever. There are also people with 30" screens. A mobile UI that fits a phone's screen but not a tablet's is considered poorly-designed. It's high time we acknowledge the same thing is true on non-mobile systems.

That being said, things are not as easy to fix as they seem. Paddings have been getting bigger as the use of contrast in UI designs has decreased, and as UI elements have gone increasingly fatter. Without either of these visual cues, the only way to "isolate" pieces of information is by keeping it apart from other pieces of information -- i.e. by whitespace. If you just took one of these interfaces and reduced all padding values by 50%, you would get sane spacing, but the whole thing would be a jumbled, unreadable mess.

(Edit: obviously, adding some visual cues like borders, if not full-on 3D frames, or at least using colours that people over 30 can differentiate on a cheap monitor in a well-lit room, does fix the readability issues, but it also gets you burnt at the design stake for being a heretic so ¯\_(ツ)_/¯ )

That might suggest that having no titlebars on systems that can display more than one window at a time isn't the best idea, either ;-)

Anyway, at least you can easily turn the title bar back on.

P.S. Am I the only one losing the fight with smaller pixels? As monitors get higher res, I keep having to increase the font size to be able to read them. Even on 4K, twenty-mumble inch monitors, I can only get one 'visible' window at a time.

Even with this fix it's still ridiculously tall --- in the space it manages to show only 3 lines: tabs, address bar/controls, and booksmarks bar, my current browser has 4 lines: a title bar (which shows the full title of the page, not these horrid cut-off fragments), a menu bar, tabs, and controls/address bar.

most people use touchscreens (phones / tablets)

...running Windows? I doubt that.

> ...running Windows? I doubt that.

of course not, windows 10 mobile is already dead. I am talking about general trend - a lot of people are fine with just smartphone and/or tablet and do not really need to use windows pc/laptop

[edit: formatting]

The problem here is that for some bizarre reason, most designers try to come up with a single solution that fits both mice and touchscreens. That should never happen. And that especially should never happen with something that is supposed to run on platforms where touchscreens are a gimmick — which they are on all those Windows laptops.

And, I mean, phones and tablets don't even run the OSes that desktop Firefox runs on. This particular redesign thus makes zero sense.

OTOH, I don't see why they couldn't have kept things adjustable with a ratio. And I miss the Status Bar on the bottom.