The performance of canvas operations in Firefox and other browsers vary greatly from version to version. Often going up. But also sometimes going down.
Here is a test page to test some canvas operations.
Currently (2012-04-05) Chromium is way ahead on my machine, an Intel i7 with a Geforce 630GT, running Linux Mint. For the fillRect test, I get:
Firefox 16: about 50.000 FPS
Chromium 20: about 480.000 FPS
Hit start to start it. It will display an OPS count (operations per second).