For 16:9 (ish) displays you have more pixels left to right than up and down, it makes sense to use up your horizontal space first when placing permanent UI elements on your screen. Still up to preference though.

A lot of websites are optimized for reading at around 1024 pixel which means many sites just give you the void to look at on both sides of a centered site (worse in naïvely scaling up all UI to the max so widscreen monitors get billboard-sized text)—so you may as well have more vertical reading space. The other part has to deal with keeping the titles readable with several open as the Latin script is horizontal. Either the titles disappear & you are left with tolerate logo favicons like Chromium or like Fx where the tabs move to vertical scrolling which is difficult to parse quickly—there’s a reason why you write your grocery list with a newline as a separator than trying to cram it all on a single line. Given the current Fx implementations using the sidebar are kind of a hack, I for one am happy to see this finally being worked on.