404 Bookmarks от Tom Schuster

With this extension you can find expired bookmarks and remove them easily. You can find all kinds of broken bookmarks, like pages that return 404 (Page not found) or other HTTP errors.

IMPORTANT
This extension might not work correctly in some situation with (ad)blockers like uBlock or Adblock Plus. Try disabling them temporarily when otherwise reachable websites show up as not working.

Expired bookmarks vs "other bookmarks"
For expired bookmarks the page returned a 404 error page, like you would often see when visiting a website that can't be found. You can be relatively sure that this is detected correctly by this extension. You should always manually check the links before deleting the bookmark, this operation can not be undone.

The category "Other bookmarks" contains all other kinds of HTTP errors, like 403 (Forbidden) or 503 (Service Unavailable), but can also indicate that a DNS lookup for this page failed. You should always make sure that these links are actually broken before deleting them.