Something must be messed up in Sync or in one "device" that is connected to your Sync account.

First thing to do is to disable Sync on all devices connected to that Sync account, as you will need to fix it on every device - otherwise the "glitch" may just propagate itself all over again.

Using your most used device, install this extension - https://addons.mozilla.org/en-US/firefox/addon/places-maintenance/ - to see if that fixes the problem on that device.

Once that is done - if it fixes that "duplication", do the same on each of your devices that were using that Sync account. If is doesn't fix the "duplication" you'll need to manually delete the "duplicates" one-by-one. Then do the same on each device.

I recommend doing that on all your devices, rather than relying upon Sync to "fix" the duplicates as the corruption may have originated on or propagated to the other devices.

Also, if you ever imported a Firefox bookmarks.html file, you will end up with duplicate folders, along with the "smart" bookmark categories. Or maybe even if you imported bookmarks / favorites from a different browser when doing a new Firefox installation.

BTW, I had that "duplication" happen once with Sync and had to fix all the devices separately; as when doing just one, the problem continued as soon as Firefox was launched and Sync re-enabled on the other devices.

Hi edmeister Thanks for the advice. I tried the 'place maintenance' add-on after unsynchronising all of my devices. I deleted all the instances of bookmarks toolbar and bookmarks bar, but some of them on each device returned immediately and others when I restarted firefox. So I am going through all of the browsers installed and removing imported firefox bookmarks in case there is some kind of loop where firefox is re-importing bookmarks from other browsers. However, if that were true then all of my bookmarks folders would be reproducing themselves, surely? Just had another look at the bookmarks dropdown menu on this pc and it is showing two instances of bookmarks toolbar and one of bookmarks bar that I deleted a few seconds ago.

How do I find out and remove any imported bookmarks from a different browser as I cannot see them, assuming that I might have done so.

At this point don't worry about "imported bookmarks" as that would usually be a one time occurrence, as it takes user action to initiate the "import".

However, if that were true then all of my bookmarks folders would be reproducing themselves, surely? Not necessarily. If you have any separators in your bookmarks the duplication could be for just one section between the separators or just "smart" items (like Most Visited, Recently Bookmarked, and Recent Tags) or the Bookmarks Toolbar Folder. The "smart" items are "surveys" of usage per device and the Bookmarks Toolbar Folder may have different bookmarks on each device.

I get rid of those "smart" items upon installation of Firefox and I keep the same bookmarks in the BTB Folder on all devices; otherwise any differences will appear after a Sync process as a separate folder - but never more than the number of devices that are different and connected to Sync.

Hi I've unsynced all my firefox browsers, deleted the "smart" items like 'Most visited', even deleted the separators where I could and still at least one extra BTB or BB comes up when I am in bookmarks or when I restart Firefox. Even when I am in bookmarks having deleted it , one BB will pop back up. That is better than 24 additional ones, but I wonder, since there were 4 extra ones today, how long before I am back to that total.