It's visually unpleasant. Boxy UI elements (notably tabs); elements of different sizes in the same container; too much whitespace in some places; generally not

It's visually unpleasant. Boxy UI elements (notably tabs); elements of different sizes in the same container; too much whitespace in some places; generally not enough whitespace in most places (bookmark folders/menus in general/the whole area around the location bar); elements that change size (the location bar); etc. I don't like customizable UIs; it seems to lead to less attention being paid to the default UI. I don't want to invest in configuring a UI; I just want it to work. Themes degrade over time as they're not maintained. Font rendering is worse than Chrome; colors are different from Chrome (I can't claim that one is right, but it's a jarring difference when comparing them, and I'm used to Chrome). Firefox doesn't have a polished UI, and its whole job is to be a UI for other content.