As of tonight, Azure canvas is on by default on all platforms! The last platform to get turned on was Linux, for which Anthony Jones did a lot of work on performance to bring Azure canvas up to par with Thebes, and exceed it in some cases. In the next cycle we will get to remove Thebes canvas from the code-base altogether.
The flavour of Azure canvas you get depends on the platform: Firefox OS, Android, and Linux get Cairo, Mac gets Quartz, and Windows gets Direct2D, if you have Vista or 7, and your drivers are up to date, and your graphics card is not blacklisted; otherwise you get Cairo.
You can see which Azure backend you are getting for Canvas (and possibly content) in about:support. You can also see the fallback backend, this is used if for some reason the preferred backend cannot be used, usually for very large canvases which are larger than the maximum texture size for Direct2D.
You can change the canvas settings in about:config: gfx.canvas.azure.enabled should be true, you can force Thebes canvas by setting it to false (for now). gfx.canvas.azure.backends contains an ordered list of backend names, for example, "direct2d, skia, cairo". Your preferred backend is the first backend in that list which your platform supports. Your fallback backend is the first backend on that list which your platform supports, is not the preferred backend, and is Cairo (which means you might not have a fallback backend).