i think this is all based on a programming error in the flash defining. opera, chrome and a few others are blacking out the errors to highlight them and firefox doesnt.
andy your problem wasnt fixed, it only occurred again and redefined the error and the screen is blacked out in a different part (that can be as small as 1 pixel). i think these errors are linked to the doubleclick and google spyware that is built in. i tried my chrome recently and noticed the screen now got a scrollbar when there shouldnt be one, indicating the "blackout" is outside the visible window and on opera i have a small black box covering the text input field partially.