Mozilla 404s '404 Not Found' pages: Firefox fills in blanks with archive.org copies
Found on The Register on Friday, 05 August 2016
The "404 No More" feature uses copies of webpages from the Internet Archive's Wayback Machine to replace 404 "not found" errors with something more useful. If you visit a link to a page that's disappeared, Firefox will fetch from archive.org a version of the page before it vanished.
To try "404 No More", Firefox users will have to install Firefox Test Pilot, a browser plugin for English-language Windows, OS X and Linux Firefox builds that lets you experiment with in-development features.
The 404 return code exists for a reason, and Mozilla is in no position to change the behaviour. Enough websites out there use custom 404 pages to notify visitors about what happened. Breaking expected behaviour is not a decision Mozilla should do. A browser has to stick to protocol specification only; but Mozilla has made enough mistakes in the past, so this will only be another one on their list.