I vehemently disagree with almost all of its design decisions, and with every new release they seem to stray even farther from sane UX.
It has hamburger menus on desktop and a lot of weird space-saving design that makes no sense on a modern high resolution display. Buttons are small, have cryptic flat Linear B icons with no labels, and the menus are hard to navigate.
It's also nearly impossible to disable all the tracking they do. Even if you do, they'll sometimes remotely change your settings, so there's no guarantee settings will stay the way you configured them. They love to talk about how privacy is important and keep building tools to disable other people's tracking, but their own data gathering is sacrosanct and must be retained at all costs.
Overall Mozilla seems to wage an all-out war on user agency, constantly trying to take away configuration options, while pushing shit nobody wants like Pocket or their themes.