A few years ago it was Safari that was the new IE, the one browser you had to go out of your way to support all its dumb little quirks. Firefox and Chrome+friends more or less "just worked"

Now Firefox is moving into that role. Except Firefox has no killer captive audience. Safari was pushed because of iOS Mobile users. Firefox doesn't have that.

So when you're a frontend dev at big corp, and you have to get stuff done now, targeting the quirks of a browser used by less than a tenthbof a percent of your userbase doesn't factor into the equation

IE was not really about quirks, it was about it abusing its dominant position to do whatever it wanted, the quirks were a symptom. Safari is still the new IE. But yes to the rest, Firefox's lack of dominance will get worse if it fails to keep up.