Paint Flashing is a diagnostic tool that shows when the browser is invalidating (or repainting) a screen region that has changed. It’s very useful for diagnosing redundant (painting too often) or greedy (painting too large an area) invalidation. Both kinds of bugs are notoriously hard to find.
Paint Flashing will paint a random color (at 20% opacity) to indicate the “damaged” area of the screen that the browser is repainting. This add-on toggles this preference: