Firefox: The default Firefox error page is visually unappealing and lacking in functionality. Fierr jazzes it up and adds in automatic reloading, search and additional buttons.
Click on the image above for a closer look.
Fierr automatically reloads the page every ten seconds to see if the error can be fixed by reloading or if the page has come back online. Clicking the search button kicks you over to Google and searches for information on the URL you're having trouble with — a button for performing a domain check at "Down for Everyone or Just Me?" would be a welcome addition. If you're in offline mode, Fierr notifies you and presents a "Go Online" button.
Fierr is a free extension and works wherever Firefox does.