Back in the day, 'all' rendering in Firefox was done using the Cairo graphics library. This was wrapped in a thin wrapper library called Thebes (Cairo is the external library which does all the work, Thebes is the internal wrapper). We would like to support different graphics libraries and we would like a more efficient wrapper layer, and so was born Azure. This is a new internal system which abstracts a lot of our drawing code. Multiple backends can be slotted into Azure, and, in turn, Azure can be used as a backend for Thebes instead of using Cairo. So, for example, our graphics stack could use Thebes on top of Azure on top of Cairo (or Skia or Direct2D, etc.). Azure has some interesting advantages over Thebes, mainly that it is a state-less API (well, except for a transform and clipping rect), Bas and roc have written some interesting blog posts about this before.
The long term plan is for Thebes to be removed, and Azure to be used directly. For now there are people implementing content rendering using Thebes on top of Azure (on top of Direct2D on Windows, Core Graphics on Mac, and Skia or Cairo elsewhere).
HTML 5 canvas uses a different rendering path from the rest of content. We actually have two canvas implementations right now, one on top of Thebes and one on top of Azure. Currently the latter is used only on Windows where Direct2D is present, and the Thebes canvas is used elsewhere. We want to remove the Thebes canvas, because having two canvas implementations is a maintenance burden and affects our DOM bindings implementation.
So, I have been working on getting Azure canvas working with both Skia and Cairo backends, the former only on Windows, the latter everywhere, with Anthony Jones. It is all about ready to land, after that, we'll let it settle for a while to ensure there are no problems, and then we can remove Thebes canvas completely. Some of the interesting bits of this work have been using Skia canvases as source for tab previews on Windows, organising different backend preferences on different platforms, and ensuring we can always fall back to a safer backend, and just getting all the complex logic in Azure canvas and the various backends right. Plus the occasional memory leak, and some interesting interactions between the layers system and Azure.
In terms of the bigger picture, this work should lead to Azure totally replacing Thebes for canvas rendering. Elsewhere, Azure is being developed into the primary backend for Thebes, so that it will be used everywhere for rendering content, rather than using Cairo directly. Then we will work to remove the Thebes layer altogether so we are rendering directly with Azure, this will be fiddly because there is Thebes code everywhere in the code base. But at the end of it all, we should have faster, more efficient code, and be able to easily plugin different graphics backends, which is a win.