ow ow ow

So sometimes, on some pages, for no reason I can discern, the whole page my browser is displaying will start to bounce. Up and down. Sometimes faster or slower. It seems to be contagious between tabs--if one tab starts bouncing, they'll all start. F11 will make it stop, but when I go back to regular window size, it's still bouncing.

Firefox 2.0.0.3, Windows goddamn Vista.

Any advice? This tab has started bouncing, too, so I have to stop looking at it...