> XBL is a proprietary technology developed by Mozilla. We are solely responsible for the maintenance and improvement of XBL. Any time spent on doing this doesn’t contribute to the evolution of the Web so effectively we don’t spend anytime improving XBL except for plugging serious holes.
What they mean is they 'own' it, the good, the tedious and the bad parts.
> Firefox has always been built with DOM, JS, CSS technologies, as the linked article calls out
I am aware of that (and imo, it was a mistake). That however does not mean that moving to more web-y stuff will not make the performance any worse.