If you use Firefox browser and bookmarked a lot of pages or sites, some of the bookmarks might have dead or expired by now. Finding them is a tough thing if you have many to go through. 404 Bookmarks is a Firefox extension which on running, detects and displays expired bookmarks, which you can remove after checking. The drawback is you need to remove each dead bookmark in the list it displayed. This could be a daunting task.

List of expired bookmarks displayed by 404 Bookmarks

404 bookmarks | AMO 

READ: Bookmark Sentry Scans and shows duplicate & dead links in your Google Chrome Bookmarks

SearchMark for Chrome allows to Search through Bookmarks and Browsing History