A point no one has brought up is that Mozilla is spending huge amounts of effort to improve their browser, the entirety of which we can take advantage of vs the situation with WebKit and how it is not used by any major browser in its entirety (yes, the js engine matters, a lot).
Related to the first point is Mozilla's work on azure. Currently in nightly you can switch drawing libraries from Cairo to skia. Yes, it's too unstable to use by default, for now, but it provides nice speedups for canvas operations, and uses less power (I.e., my fan didn't need to crank on). When we make the move to Wayland we're going to be relying on glamour to provide our 2d acceleration and skia has a far better story when it comes to gl acceleration.
Yeah, we're still stuck with Cairo, but there's no reason not to make the situation better where we can.