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.
In full screen mode, minimize the tab, address, and bookmark bars into the top of the screen. And personally, it bugs me how obscure it is to work with bookmarks (yes, I still use them).
It's still my daily driver at work and home but those are two things that always bug me.
