Internet is an online database of webpage’s and other stuff surfed by people all over the world, when ever you browse any website you surf different pages on that website and some pages might give you an reverse error code like page not available, page removed, page expired or any other 404 kind of error, well this is due to in availability of that page well still you have the option to view the cached page option from Google that will show you a simple screenshot of that page when it was available.
But we have another Firefox add-on called "404-Error ?" which basically fetches your page from archive.org which is known as archived page, so its similar to what cached page look’s like and your website will be back online.
Now this extension works when ever it detects the not found error in the HTML title tag, if that thing is not found it will not automatically show you the archived page, but you can still access that archived page by clicking on the add-on button in the toolbar.
So this is pretty useful extension at times when you have no option to see that disabled or expired page, well thanks to Lazar Kovacevic for developing this add-on for Firefox.