I downloaded the FF4 RC this morning, and immediately used it to test our medical app which runs in a full screen Silverlight session. The app appeared to render properly, however the mouse tracking was not correct. The horizontal location was accurate, however it thought that the vertical location was actually much lower. In other words, if I hover the mouse over a button, it does not highlight, nor can I click it. However if I move the mouse pointer up a few inches above the button, I will see it highlight and can click it. This effect is much more noticeable on something like a grid where the rows highlight as you mouse over them. The highlighted rows are several inches BELOW where the mouse pointer is on the screen. Everything on the page still works properly in other browsers, and in FF 3.x.