Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0b7pre) Gecko/20101005 Firefox/4.0b7pre When opening a new tab, if you look at the top left of the tab while it's opening you see a graphical glitch. To best describe this I have captured a video, see it here: http://www.youtube.com/watch?v=9qklB9ZXcOw Focus on the top left of the tab, it looks like you see a squared off corner from some kind of element.
Simply looks like that the rounded borders aren't applied for the transition. Does that only happen on OS X?
Most visible on OS X, happens on all platforms.