Here a brief description from the Link Evaluator page:
Link Evaluator is a Firefox extension designed to help users evaluate the availability of online resources linked to from a given Web page. When started, it automatically follows all links on the current page, and assesses the responses of each URL (link). . . .
After each link is checked, it is highlighted with a color based on the relative success of the result: green for fully successful, shades of yellow for partly successful, and red for unsuccessful.
It requires Mozilla Firefox version 1.5 (or later).