Earlier quoted context omitted.
The new "chunky" layout and UI elements are not really an improvement, and a lot of users have been unhappy with this. Despite plentiful feedback about this during beta testing Mozilla have basically opted to force it through anyway. UI changes in general are something to expect, but if you're just going to disregard user feedback then you can't claim to respect users.
Equally a lot of users are happy with it. Should those users have been disregarded instead?
The proper solution to this would be to have different modes if this is so critical. To their credit they did put in a "compact" mode after beta, but this is explicitly "unsupported" and even then still has more padding than "large" mode in the old UI.