The moment you said put "great writeup" and "hacks.mozilla.org" in the same sentence, I had a feeling it would be Lin Clark. Highly recommend her articles, I've learned a ton about Firefox's internals from her.
I'd be curious how the power usage compares between this GPU rendering and the old CPU painting / compositing rendering. I'd expect it to save power but could also see it going the other way, particularly for webpages with minimal changes from frame to frame.
At present power usage tends to be worse with WebRender, but I'm working on that with OS compositing (planeshift), and Glenn is working on that with picture caching. I expect it to be an overall energy consumption improvement over the old graphics system once this work is complete, due to reduced overdraw and better use of OS composition.
Also a great read for anyone interested in long-form technical articles that try to serve a wide range of reader types.