Essential Firefox Add-On - ErrorZilla

Have you ever clicked on a link only to get brought to a page that's not available?

[ErrorZilla] Implements a useful error page when a website cannot be reached. Will add "Try Again," "Google Cache," "Coralize," "Wayback," "Ping," "Trace," and "Whois" buttons, along with the Firefox logo to the error page when a website isn't found or a web server is down.

This is a great way to figure out if a site is permanently unavailable as well as track down what was originally on it.