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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).

They get the benefits of cross platform code, but now use something that developers are more familiar with, and the can get rid of a lot of code. Big win, I think.