Firefox is very busy at the top. It has like 2 or 3 layers, one for the tabs, another for the bookmarks. This person removed the extra crap and gained some space on the top of their browser window.

They got MS Edge like vertical tabs going, which after also using for nearly a year now I will say should be the default setup on wide/desktop formats.

We got more horizontal space than vertical in 16:9 and so on, so it maks no sense to have a second horizontal bar taking up precious vertical space when we can trivially use vertical tabs to use up the abundant space instead. More so because most websites use only a tiny fraction of the horizontal space in their layout anyways (case in point, Lemmy renders the same width whether I got vertical tabs or not).

Ever since MS Edge came out I've been looking for this in other browsers, and at least in Firefox I can do it myself, I just wish it was an official thing.