Don't you find it irritating when a Web page you bookmarked or favorited returns a 404 error on a subsequent visit? Or when a Web site is temporarily down? Firefox extensions Resurrect Pages and 404: Page is Not Found? Now it will be! can help in such scenarios. While Resurrect Pages relies on several popular page cache sites, 404: Page is Not Found uses the Wayback Machine at Internet Archive to serve the dead pages.
To install each extension, click the shiny green Add to Firefox button on its homepage. After the customary browser restart, you'll find the extension available from the right-click context menu. You can even customize Firefox's menubar to sport icons for these extensions if you like. Right-click the menubar and click Customize. Scroll through the available icons, and when you spot the icons for the extensions, drag and drop them onto the menu bar.