It looks like the scrolling in Firefox is implemented in screen-height tiles, and when he's scrolling it's picking a smaller and smaller portion of the top tile (as the top portion of the tile is scrolling out of the window) but not updating the bottom coordinate of the rectangle it's supposed to render to (so instead of moving up, instead the smaller and smaller portion gets stretched out to fill the window). Then when the top tile is supposed to be completely off-screen, it jumps to the second tile in one go.