The internet is a constantly changing place. There's no guarantee a page that existed a few years -- or even days -- ago will remain that way. Depending on how desperate you are, you can always give Google's web cache or the Wayback Machine a go, but wouldn't it be nice if your browser did this for you? Mozilla thinks so.
A recent addition to Firefox's Test Pilot program (which allows users to try out new browser features up for consideration), "No More 404s" will attempt to retrieve a dead or missing page using the Internet Archive's Wayback Machine.
It's certainly territory well explored by add-on developers, but this looks to be the first time a browser vendor is considering incorporating such functionality directly.
As gHacks' Martin Brinkmann explains, the experimental feature only responds to 404s, so timeout or server errors will still be presented. And rather than instantly teleport you to a cached version, you'll get a sliding notification from the address bar.
As long as it doesn't obfuscate web errors in general, it looks like a nice convenience and I wouldn't be surprised if it found its way into Firefox's stable release channel. For now, hit up the Test Pilot page to give it a go today.