The only thing slow about Canvas I've found is when the canvas area needs to be resized : i.e. canvas.height or canvas.width changes. Drawing itself hasnt been an issue for me.
Try changing fonts often on Firefox. That's ludicrously slow. To the point where anyone doing lots of text on a canvas that might need to support Firefox will need to take special measures to get decent performance (caching, lots of caching, and ordering updates to minimize font changes)