It's likely that you have dozens--perhaps hundreds--of Firefox bookmarks in your Bookmarks menu, especially if you've been using Firefox a long time and have an uninterrupted upgrade path across all those years. It's difficult to find useful sites among ones that you don't need anymore, and picking out the bad links is tiresome. Bookmarks Archiver, a tiny and free add-on for Firefox, attempts to solve this problem by allowing you to archive or delete bookmarks that you haven't visited in 90 days or more. Unfortunately, it's only moderately successful as a solution.
Bookmarks Archiver launches an annoying popup each time you start up Firefox with expired bookmarks. (The 90-day number can be changed, of course.) Bookmarks Archiver then lets you choose to archive or delete said bookmarks, or automatically archive or delete all expired bookmarks. This automatic function works only that time and can't be set in options, so you'll see the popup again the next time a bookmark expires.
Bookmarks Archiver has trouble with bookmarks that are organized within folders. When selecting "always archive expired bookmarks" on one bookmark within a folder, the add-on ignores the others in the same folder. When selecting that option on a non-foldered bookmark, it correctly archives the remaining non-foldered bookmarks.
It's also worth noting that the concept of bookmarks is somewhat outdated. Web browser power users usually RSS subscribe to everything that moves, and use sites like Digg to help them find cool things. Firefox users usually take advantage of the browser's smart recent history dropdown to find their favorite sites. Bookmarks just seem so 20th century.
That said, if you're willing to overlook the lack of an automatic feature that works correctly, and want the ability to clear away unneeded bookmarks either with or without deleting them, Bookmarks Archiver will do the trick.