I don't know what's going on with ESR 10.0.4 (there is no tagged build as of this writing), but 12 is signed off and has build tags, so we're shipping. I will build ESR 10 tomorrow regardless of whether there are build tags or not because we need to get basic testing by you, the basic testing basic audience, basically before we basically ship.
You will notice one particular difference between Cairo-based canvas and Azure CG canvas, depending on the fonts you have installed and their supported faces. If your Helvetica, say, lacks an italic, then if you ask for italic in that font you'll just get roman, whereas Cairo would try to simulate it. Strictly speaking I think CG has the correct behaviour here, and actually the CG backend works better with weird fonts in pdf.js as well that don't even display with Cairo.
Using CoreGraphics also introduces a modicum of hardware acceleration, which works on both 10.4 and 10.5. Stretching the canvas (such as in this GameBoy emulator when you press ESC) uses hardware to stretch it rather than software, and certain other CoreGraphics operations can also be accelerated by the graphics card. The proof is in the benchmarks. Not only compare TenFourFox 12 against 11, but try turning gfx.canvas.azure.enabled off and see what it does to your Peacekeeper scores. CoreGraphics accelerates these operations up to 60%. I can't wait for them to expand it to other components in the graphics stack, particularly SVG and HTML5 video blitting.
Other than that, 12 is a pretty blah release. There are some new CSS and DOM properties, but that's really it. Memory usage seems a bit worse due to a new font shape cache they are using, but this improves text-heavy pages, so it's hard to call it a bad tradeoff. The big changes in tab control and SPDY-by-default don't start appearing until Fx13.
Release notes and architecture builds (watch for 10.0.4 probably tomorrow evening Pacific):