I have most current Firefox on Windows 10 System. While using Firefox, all of a sudden, without any action by me, my bookmarks, both Bookmark Menu and Bookmark
While in Firefox, Bookmarks Menu (also Bookmarks Toolbar) will start duplicating; no action on my part - just starts adding bookmarks. Some are empty folders of same name as existing folders which have individual bookmarks, some are duplicate folders with same individual bookmarks in them. This first occurred on a desktop. Browsed internet, learned of Bookmarkdupes (with the 2 stars). Installed, cleaned out bookmarks, and started over building a bookmarks menu. Via Sync, apparently, laptop then had the new bookmark set, as did mobile devices. Some couple of weeks later, while using laptop, bookmarks started duplicating again. From about 100 bookmarks, I wound up with over 4,750. Then, that synced to my desktop and mobile devices. How do I stop the duplicating. Again, browsing the web, some support pages pop up which seem to address the issue. I have already done the Verify Integrity suggestion. But what is Firefox Quantum, what is Foxmarks, now Xmarks. References to those and suggestion don't use Sync. Sync is valuable if you have multiple devices, but worthless if you have to turn it off.
what is Firefox Quantum
You are running Firefox Quantum, a renaming after version 57 as Firefox is a multi-processor browser meaning uses more than 1 core of your computer as can be seen in Task Manager. It is Multi-Thread since ver 58.
Foxmarks now Xmarks
Was a Bookmark Manager / Sync for Bookmarks
Sync is valuable if you have multiple devices, but worthless if you have to turn it off.
It is not made to be used on a single device. Running Sync 24hr - 7 seems like a security accident waiting to happen especially wireless. Never used it.
If you know which device is the one that initiates the issue then leave the others alone.
Start by cleaning them out :
Then Please try a Refresh but Note that this does more than uninstalling and re-installing does. Normal uninstalling does not remove some preferences which may have become corrupt. Please Refresh but do this 1st:
Do not copy the problem over again to the new profile.
As far as that duplication of bookmarks goes, in Troubleshooting Information the Places Databse > Verify Integrity button should be used to fix that duplication on the desktop / laptop PC's. But only after each device is Disconnected from Sync and the Firefox Account password is changed, which clears the all your Sync data from the Sync server to get rid of the corruption there. Doing those things separately or out of order just ain't gonna fix it.
And it's a waste of time to delete the duplicates in one Firefox installation while leaving that device connected to the Sync server; massive changes like deleting or rearranging many bookmarks in one sitting or doing it too fast can cause corruption problems in Sync. Disconnect before re-organizing your bookmarks, when it is more than a few changes; I watch the CPU Monitor that appears when the Windows Task Manager is open, with the Task Manager minimized so I can see the CPU load on my 4 cores and I stop moving bookmarks when they get close to 100% and resume when Firefox has "digested" the changes that I just made & the CPU load drops dramatically. I have actually lost bookmarks when I pushed Firefox too fast when rearranging my bookmarks, and that with not even using Sync with that Profile.
Once each desktop / laptop has been fixed using the Verify Integrity button, you can connect the first device and wait awhile before connecting any more devices. Sync does take awhile when the server needs a full loading of data, and time that takes varies depending upon the load on the server and how much data you have that needs to be sent to the Sync server.
Then slowly add the additional devices to the Sync circle, and check to make sure that when the 2nd and subsequent devices are added that the corruption doesn't return on any already connected device before proceeding the adding the next device - the 3rd,4th, etc. If something is still screwed up or you push it too fast, you'll need to start all over again - based upon the feedback from users who have asked for help here.
I don't have a "mobile"device to use on Sync, so I can't experience that corruption first hand; but with a search I have done of past forum threads about Sync and multiplying data that I have found going back to April / May 2015 I am convinced that this problem with Sync bookmarks corruption / multiplication started about the time that Firefox for iOS went Beta and the Sync server (and probably Sync in Firefox) was changed for the two "mobile" platforms; which includes Firefox for Android. And from user feedback here neither "mobile" platform seems to be affected with the duplication problem, it only affects the desktop / laptop devices. I have seen posts of anywhere from 2 or 3 sets of duplicate bookmarks to dozens and in a couple of cases over 100 sets of duplicates bookmarks with users who didn't see that happening.
edit with added information Yes that is a lot of work, so if you are considering using a different synchronization app now might be the tine to try it. But with duplicated bookmarks already they probably won't fix those; they would probably transfer the duplication to the other app.