The browser UI was already rendered by Gecko, that's how XUL and XBL work. It's just that it's rendered by parts of Gecko that aren't used by anything else on the web, and so in honesty those paths probably had less optimization, not more.
That sounds like a great thing, because the Web is where we applaud people if their site renders in a second, enough time for a modern processor to render a full fricking playthrough of the original Doom.
As for XBL: as far as I understand, it was closer to Custom Elements - defined at XML/JS layer instead of with native code. In fact it was so similar that FF developers made a tool that tried to automatically convert an XBL binding to equivalent Custom Element (similar to Python's 2to3).
Proud that you need to use your HTML/JS engine to render your UI instead of using faster native code? I mean you have the opportunity of getting rid of XUL and instead you change it... for something similar?