Curious about the CPU usage of canvas apps on older systems, I ran this in FF and Chrome. (I'd classify the system I'm on at the moment as a mid-tier business PC from 2008.) FF maintained a CPU usage mostly in the 33-40% range and Chrome in the 18-23% range. CPU usage would spike up to 75-90% in FF, whereas the largest spike in Chrome was only 40%.

It's just one data point, but I was impressed with how much more efficient Chrome was than FF.