[pcwalton]

I'm putting "GPU support" in quotes because "GPU support" is such a broad topic that it's impossible to pin down precisely what you might mean by it. There aren't any areas I'm aware of in which Chrome uses the GPU whereas Firefox doesn't. If anything, WebRender makes it the reverse, though since Chrome uses Skia-GL it's more of a matter of which GPU features a browser uses rather than whether a browser uses the GPU (in particular, WebRender uses the Z-buffer and the early-Z functionality, which are quite an improvement).

I will say that, in my opinion, WebRender is generally better optimized for modern graphics hardware than Skia-GL as used in Chrome, due to the reduced overdraw via aggressive use of the Z-buffer and better batching.

Often times performance bugs are just that—bugs. The existence of performance bugs doesn't necessarily indicate that "the whole pipeline" in one browser is better than the other. Browsers are broadly quite comparable in terms of the script-layout-painting pipeline these days. In the case of Firefox, I'm personally confident in the IonMonkey-Stylo-WebRender trio.