Layout paint flashing in Firefox « JAWS
http://msujaws.wordpress.com/2012/02/01/layout-paint-flashing-in-firefox
Starting in #Firefox 11 (currently in the beta channel), there is a hidden #preference (nglayout.debug.paint_flashing) to enable what we call “paint flashing”.
Whenever the layout engine determines that a region of the browser requires repainting, the region is tinted with a random color value. Regions experiencing heavy paint flashing can turn your browser in to a fun rave, but also show web developers (and browser developers too!) areas of the code that aren’t being as optimal as possible.