Firefox only (Win/Mac/Linux): Firefox extension Bookmarks Archiver cleans out those old bookmarks that you never use, but still have cluttering up your bookmarks menu.
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Once you've installed the experimental Firefox extension, you should take a trip into the add-on's options panel to set the number of days to check, and whether you want the bookmark archiving to prompt you or not. Once you've configured the settings to your liking, whenever you restart Firefox you'll be prompted (or not) to archive or delete the bookmarks. Since the extension relies on the history feature to determine the last time you've visited a bookmark, you'll need to have visited the bookmark at least once to trigger an archive, but it's an interesting add-on definitely worth a look.