"Firefox out of the box indeed does not cause broken websites" .. for you.

I regulary ran across something that does not work or has a broken style. And I know how to turn UO off.

And why should that be a surprise? FF has way less manpower than Chrome. (And even fired lots of engineers, to raise the CEO bonus)

Chrome leads the way and probably the vast majority developes for chrome and with chrome dev tools. So most of the time FF works, but not always. And performance is just worse, but not noticable on a desktop and on mobile it is offset by the working adblock. Meaning perceived performance is usually better, because ads are blocked, unlike in chrome mobile.