Rated 4 out of 5 stars
I was tired of losing bookmarks. Now and then I would add a page to the bookmark bar (normally to read it later, often by dragging by mistake) that I had previously saved and classified. Firefox had silently removed the original bookmark so, as soon as I remove the last addition, the page is gone. Once more, data loss as a feature.
This add-on overcomes this "feature" while gracefully allowing me to opt-in in a per bookmark basis.
The only drawback is unintelligible text of the duplicate bookmark dialog ("Do you bookmark of duplication url?") but I can't figure out how to contact the author (the add-on site is in Japanese).