Reported from IRC. Loading a progressive JPEG on a current nightly is suboptimal. After a frame is shown, the image is cleared and then the next frame is drawn (or the image is cleared when the next frame starts?). Essentially, this means that the closer a pixel is to the bottom of the image, the more time it spends being blank. Chrome seems to do the right thing here. Either it's not doing any incremental display, or it's retaining the previous decoded frame as it displays the next frame.
Blah. Using the wrong term -- not "progressive jpeg", but rather "motion jpeg".
And now I can find bug 625012!