Alexey Gavrilov has written a piece comparing the animation performance of WPF/E, WPF, DHTML and Flash. One remark based on the article: we've got a known bug slowing down the performance on Firefox and we're current working on a fix for that. 

Here's the performance results from Alexey's article:

Here are results I got testing with 16 balls on my Pentium M 1.7 laptop:

Browser DHTML Flex WPF/e WPF
IE 6.0 56 61 84 99
Firefox 55 52 58 -
Opera 9.01 94 50 - -

All the tests can be found here.  Click the image below to jump straight to the WPF/E version of the test.