[Ygg2]

> more powerful XUL-based

At the time of conception, it was pretty great. However, that much customizability came at two expenses.

1) Web devs can't use XUL, because it's its own little cosmos, you as Web dev couldn't mod Firefox UI

2) The XUL was legacy software and super hard to optimize.

[pessimizer]

Repeal and replace would have been fine, but instead we got repeal and here's a mobile OS. And new extensions can't mess with our look and feel.