I've tested this on a couple of iPhones, got from 44 fps (iPhone 2G, OS 3.1.2) to 87 ! (iPhone 3GS).

Desktop results around my testing vary from 64 fps on Firefox 3.5.4 and 65 fps on IExplorer 8.0.7600 to 228 fps Chrome 4.0.223 and 250 fps Chrome 3.0.195.27, all on the same Lenovo T61p with Windows 7.

It's quite impressive for an handheld ARM WebKit device to get better javascript performance than Firefox on a dual core x86 machine. Safari on MacBookPros got from 88 to 98 depending on the exact hardware.

This shows that impressive performance of Javascript on ARM is possible, and that Safari is better tuned on mobile devices than Mozilla.