Since I upgraded to firefox 8, I keep getting brief black boxes flashing every time I mouseover almost anything on any web page. I have all of my extensions disabled and very minimal addons. The black box also appears every time I type over the entire text field I am typing into and sometimes over images with mouseover events on them (which is just about everything). Gmail in particular is a hot mess with this problem the whole screen goes black for several seconds when I do almost anything in it.