Your bookmarks are, too!!!!
If it's a 'Bookmarks Manager' you're looking for, one that manages your bookmarks, specifically, many don't provide anything to actually help you to achieve what you could ever call a managed collection of bookmarks....
BookmarkOS certainly does, alphabetize + a duplicates tool are both standard issue so you are, as I said, sorted!!!!
This add-on is just another on the list of handy features, it makes adding a new bookmark as simple as you like + the icon sits on your toolbar, or bookmarks bar, wherever you decide......
It all adds up to exactly what I was after, something to manage my bookmarks + keep them that way, which it does with ease so, as I've said, twice, but in case I haven't got my point across? You're sorted, + so are your bookmarks!!!!!