It's not just performance that was the issue. XPCOM gives you access to basically the entirety of the browser internals. There's no designed API, the implemen

It's not just performance that was the issue. XPCOM gives you access to basically the entirety of the browser internals. There's no designed API, the implementation is the API. Which means that you can't change the browser internals, unless you're willing to break addons, or you perform an audit to figure out which plugins will be broken and you get them to update first. Basically all refactors took months and months and months because of this. There was no way to address the accumulating technical debt.