To recap, we previously had two implementations of canvas (not including WebGL canvases which are different). One using our older graphics library, Thebes, which is a thin wrapper over Cairo; and one using our new graphics library, Azure, which supports multiple backends (Cairo, Skia, Direct2D, and Quartz/CoreGraphics currently). To get to this stage we needed to get Azure canvas working properly on all platforms, which was finally finished off last cycle. Now that Azure canvas works properly, we can dump Thebes canvas, which removes a whole bunch of duplicated code, and thus duplicated effort in maintaining two implementations. Also, it means canvas always uses the new DOM bindings.
In the code, nsCanvasRenderingContext2D has gone and nsCanvasRenderingContext2DAzure has been renamed to mozilla::dom::CanvasRenderingContext2D. The various bits of XPIDL for getting a Thebes canvas rendering context has also gone.
If you are testing nightly, let us know if you spot any new bugs with canvas (including when printing, which uses (sometimes or always, I'm not sure) a canvas).