They won the first one, breaking IE's monopoly. Then they lost the second one to Chrome, which as of 2015 attained >50% browser market share and only keeps going up. Just being good isn't good enough, it's needs to be so much better that trying Firefox and then going back to Chrome makes it painfully obvious that Chrome is not just "not as good" but outright "bad".

And that's a really, really hard thing to pull off.