> Touching the DOM in Firefox is a terrible bottleneck, for example
Yep. Speedometer 2.0 shows this pretty well. It's 25% faster in Chromium for me on desktop, and over 100% faster! in mobile Chrome than Firefox for Android. Mozilla have done a remarkable job of catching up on raw javascript engine performance and the speed of the renderer, but they have a ways to go yet in other areas, probably none more so than bringing recent improvements to mobile.