What bothers me, are dialogs and windows not written in html and css (not sure what it is, maybe XUL?). Chrome basically eats it own dog food = everything is html. Firefox still has settings done in non html way. It may sound marginal problem to you, but IMO browser should be showcase for html and html should be used everywhere. Also amount of popup dialogs/windows should be reduced. Again settings, about, ... should be html page, instead of popup window, which cannot be until it's rewritten as html page (so it's related to my first point).
UI elements written in HTML are hella buggy. For example, Chrome's hamburger menu (I don't know if it's written in HTML but it sure feels like it):
1. Spacebar doesn't select items
2. Key equivalents don't work while it's open
3. Cut/Copy/Paste often does not work, depending on where the keyboard focus is
4. Inapplicable menu items don't disable properly
5. The caret continues blinking in text fields even though keyboard focus is in the menu
etc etc. This sort of basic bugginess is embarrassing and entirely avoidable by using the native platform controls, instead of re-inventing them poorly.
FYI: I use Firefox for idealogical reasons.