When it comes to bookmarks no. When it comes to any merge you take your chances.

Depends if want to take chances. Never work with Originals....... Not saying how you are working on them. HTML would be good way I guess : https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer I am still working with my bookmarks from 95. Just overwrite and copy off. HTML is wonderful

Can see what comes up in here using your words : merge bookmarks https://addons.mozilla.org/en-US/firefox/extensions/ and here https://mozilla.github.io/extension-finder/ bookmark will do

Note they want lots of things and can put context menus in right click menus or...

Please let us know if this solved your issue or if need further assistance.

I've seen one de-deduplicating extension but I haven't tried it on a large number of bookmarks. Before letting it run, make sure to set the search to check for duplicates rather than to check whether all the addresses are valid.

https://addons.mozilla.org/firefox/addon/bookmarks-organizer/

Thanks for the answer, unfortunately, it stopped to scan/freezes at scanning ~50th bookmarks.

Copy/Paste Bookmarks into the Search Bar you pick your poison as there are others, also try merge https://addons.mozilla.org/en-US/firefox/extensions/

Possible would have had more done doing copy/paste html

Please let us know if this solved your issue or if need further assistance.

I am curious why you have so many duplicates bookmarks & folders?

Since Firefox 3.0 came out, Firefox doesn't save a duplicate of an already existing bookmark thru the "normal" save action in Firefox.; the new for 3.0 Tags feature was created to get around the need to save the same bookmark multiple times, such as in different folders, by attaching multiple Tags to classify each newly saved bookmark as it is being saved. Never liked that feature myself, and never used it; I just stuck with my folders and multiple bookmarks as I felt necessary using the "drag'n'drop" method to save the bookmark multiple times to different folders.

There are ways to "game" the save system, and when the user imports a set of bookmarks via HTML any duplicates will be in Firefox.

But a corrupt places.sqlite file can also have unintended duplicates of bookmarks and folders; and if the user is using Sync the duplication would get propagated to the other devices using that Sync account.

Should that occur, all the fixes the user does very well may get undone or the corruption could get worse. All devices should be disconnected from Sync before things are made worse thru the Sync process.

The user needs to start "fixing" by verifying the integrity of the Places database in Help > Troubleshooting Information down the page a ways to Places Database > Verify Integrity ----- on each device to fix the actual cause of the duplication.

Then change the Password for the Firefox Account / Sync account before Connecting back to the Sync server; that will wipe the corrupt data from the Sync server and allow a fresh data into the Sync process.