Visual Bell by Nicolas B. Pierron

This addon will let you select an area in the current page after clicking on the icon of the addon.

Once selected, any modification to the selected elements, such as adding text, will get notified to you visualy, by drawing a red outline which bounces a few times around the modified element which was selected previously.

This addon is useful if you require a noise free environement, such as in a meeting, but require your immediate attention on changes of elements which is in your peripheral view.