Rotten от Sean Reed

Rotten is a simple extension that roots out link rot. It collects all the links in an active webpage, displays them in a sidebar, and highlights any links that do not return a status code of 200. For an example use case, you're working on your website and want to make sure that all the links in your latest published blog post are still working properly. The extension will report the status codes for all the links and highlight red any links that return a status code of 404, for example.