On the MessageBox dialog sample page, the MessageBox on the simple Confirm looks strange when first loaded. Looks like the title is cut off and everything is rendered too high. When you actually move the dialog with the mouse, it looks like it lays out everything ok.
I also am seeing this in fact if you click anywhere on the viewport it snaps into place. Works fine in webkit. I'm using FF 3.613. After click you have to refresh the browser to see the bug again.