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