Firefox 12 is the first release that we use the new CoreGraphics backend for canvas. This brings a host of performance improvements, that largely come from removing overhead and semantic mismatches between HTML canvas and CoreGraphics.
Here are some examples:
GUIMark2 vector test 6.29 fps to 6.63 fps
GUIMark2 Bitmap From 17.62 fps to 22.9 fps
Fish IE goes from a high quality but embarrassing 7fps with 10 fish to 48fps with 250 fish.