The main issue I've had with their new UI is the massive size of everything, it's fair enough for fat finger phone displays but is annoyingly wasteful for any regular sized desktop. This is partly due to the increased padding as per the article, but also because they removed the "compact" UI density option. However you can bring this back under about:config browser.compactmode.show Then go to menu > more tools > cust…
It's strange, I much prefer a substantial amount of padding to my interfaces. Having a good amount of padding lowers the visual noise/clutter and gives everything room to breathe, which may not give a specific practical advantage, but makes me feel less anxious looking at it.
It, in fact, gives a specific practical disadvantage.