I made this suggestion a few years ago, but I will offer it again:
I make an exported saved copy of my bookmarks every few month (for 26 years), and have many duplicated websites bookmarked spanned across many files. Add the ability to compile several saved backup bookmark files, consolidate them into one file (checking for 404 file not found and redundancies, then keeping the newest bookmark) to save the new backup bookmark file.
backup 1 (2021-05-01) - site A, site C (2021-01-01), site G, site J (404 file not found in current search after backup date)
backup 2 (2023-03-05) - site C (2023-02-01), site E, site Z
new consolidated backup (2023-03-17): site A, site C, site E, site G, site Z. All of which have been scanned, refreshed and updated to be valid websites accessible when consolidation was performed.
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Thanks for sharing this, @User-1962-07-15! If you have a chance, I recommend updating the title of your idea from "Bookmarks" to something more clear and descriptive of the request. This will help the idea stand out and make it easier for others (community members and Mozilla employees) to find it. Hope this helps!